Chapter LVIII: Appendix: G (3)

Polarity, organic, of physiological units, =I=, 226, 315, 317, 332,
350–1, 701–6.

Polyatomic compounds (_see_ Chemistry).

_Polychætæ_, anomalous development in _Myrianida_, =I=, 361.

_Polycytharia_, integration, =II=, 90, 124.

_Polygastrica_, aggregation, =I=, 586.

Polymerism: of compounds, =I=, 9, 11, 25;
nerve tissue, =II=, 356.

_Polypori_, symmetry and environment, =II=, 139.

Polyps (see _Cœlenterata_).

_Polyzoa_: size, =I=, 140;
multiaxial development, =I=, 165;
structural indefiniteness, =I=, 173;
functional differentiation, =I=, 202;
trochophoral kinship, =I=, 447;
integration, =II=, 93–4, 96, 124;
symmetry, =II=, 194, 207;
vascular system, =II=, 340;
gemmation, =II=, 444.

Poor Laws, and natural selection, =II=, 532.

_Population, A Theory of_, =I=, 265, 577–601; =II=, 411.

Potato: simulated growth, =I=, 136;
vicarious function of tuber, =I=, 209; =II=, 255;
sub-species, =I=, 302;
dye absorption, =II=, 279.

Preservation: fertility and self-, =I=, 581; =II=, 423, 430;
nutrition, =II=, 493.

“Progress; its Law and Cause,” theory of species differentiation,
=I=, 568.

Projectiles, factors in flight of, =I=, 450–1.

Proteids: metabolic function, =I=, 67, 68, 69, 72, 76;
complexity of molecule, =I=, 122.

Protein: evolution, =I=, 23, 24;
isomerism, =I=, 700, 703, 704.

_Proteus_, degeneration of eye, =I=, 613.

_Protodrilus_, intestine segmentation, =II=, 125.

_Protophyta_: internal movements, =I=, 56;
limit of growth, =I=, 138;
development, =I=, 164;
structure, =I=, 173, 181–3;
self-mobility, =I=, 175;
individuality, =I=, 245;
multiplication, =I=, 270, 276, 279, 581, 584–5; =II=, 439, 462;
genesis and nutrition, =I=, 295;
unicellular, =II=, 21;
central aggregation, =II=, 24;
symmetry, =II=, 134;
tissues, =II=, 244, 249;
primary differentiation, =II=, 385;
primordial type, =II=, 398;
symbiosis, =II=, 400.

Protoplasm: self-increasing function of primordial, =I=, 63–4;
plant metabolism, =I=, 65–7;
complexity, =I=, 122, 253–5;
differentiation in simple organisms, =I=, 182–3;
continuity and inter-circulation, =I=, 190–2, 371, 629; =II=, 21,
620;
“streaming,” =I=, 253;
structure, =I=, 253–5.
(_See also_ Cell.)

_Protozoa_: inorganic components, =I=, 17;
locomotion, =I=, 58, 175; =II=, 14;
vital changes shown by, =I=, 94;
limitation of growth, =I=, 138;
development, =I=, 164;
structure, =I=, 173, 181–3;
incipient differentiation, =I=, 198, 391; =II=, 299, 309;
multiplication, =I=, 270, 276, 279, 280, 582, 584; =II=, 442,
451–2;
genesis and nutrition, =I=, 295;
distribution, =I=, 396;
parasites infesting, =I=, 427;
Weismann’s hypothesis of immortality, =I=, 637;
“spontaneous generation,” =I=, 697–701;
non-nucleated, =II=, 20;
primary aggregate, =II=, 86–7, 124;
progressing integration, =II=, 89–91, 124;
symmetry, =II=, 186;
primordial plant-animal type, =II=, 397–8;
symbiosis, =II=, 400.

Protyle, hypothetical chemical unit, =I=, 22, 23.

Pseud-axial development, vegetal, =II=, 28–9, 30.

Pseudo-foliar development, vegetal, =II=, 26–8, 30.

_Psychidæ_: parthenogenesis, =I=, 275;
sexual dimorphism, =I=, 683.

Psychology: reasoning and definition of life, =I=, 81–8;
correspondence shown by _recognition_, =I=, 95;
contrasted with physiology, =I=, 127;
departments of, =I=, 127–8;
vicarious function, =I=, 209;
waste and repair in sensory organs, =I=, 217;
sensory adaptability, =I=, 229, 231, 232;
inheritance of sensory defects, =I=, 306;
musical talent, =I=, 311–2;
intellectual progress and special creation hypothesis, =I=, 417;
special creation a pseud-idea, =I=, 420, 429, 433, 554;
legitimacy of evolution hypothesis, =I=, 433–5, 439, 554;
embryology of ideas, =I=, 450, 457;
persistent formative power unrepresentable, =I=, 492;
E. Darwin’s and Lamarck’s theory of desires, =I=, 494;
natural selection and brain evolution, =I=, 553;
genesis and cerebral activity, =I=, 594; =II=, 512–4, 516–9, 530;
heredity and distribution of tactual perceptiveness, =I=, 602–8,
646, 665–6, 672, 692;
inconceivability of the negation, =I=, 675;
vitiation of evidence, =II=, 88;
repetition and perception, =II=, 143;
differentiation of sensory organs, =II=, 317–20;
differentiation of nerve tissue, =II=, 355–61;
functional integration, =II=, 376;
also integration, =II=, 380–2;
equilibration of nerve discharge, =II=, 393;
human fertility and nerve development, =II=, 466, 532;
future human evolution, =II=, 523–5, 527;
human evolution and genesis, =II=, 529–31;
future mental development, =II=, 535;
origin of vertebrate type, =II=, 598–600.

_Pteridophyta_: size attained by, =I=, 138, 139;
homologies, =II=, 80–1, 82;
frond surface differentiation, =II=, 260.

_Pteropoda_: bilateral symmetry, =II=, 201;
dermal respiration, =II=, 310.

Ptyaline, metabolic function, =I=, 69.

Punjabis, inheritance of acquired osteological peculiarities, =I=,
689.

_Pyrosomidæ_: phosphorescence, =I=, 47;
integration, =I=, 588; =II=, 97.

Quagga, telegonic transmission of markings to offspring of mare,
=I=, 624, 627, 646.

Quills, development, =II=, 314–6.

Rabbit: activity and muscle colour, =II=, 365;
over-running checked by weasels, =II=, 405;
expenditure and genesis, =II=, 472.

Radial, definition, =II=, 148.

_Radiolaria_: unicentral development, =I=, 163;
secondary aggregation, =II=, 88;
symmetry, =II=, 187.

_Radula_, development of roots from leaflets, =II=, 34.

_Rafflesiaceæ_: homogenesis, =I=, 272;
tissue differentiation, =II=, 274;
nutrition and genesis, =II=, 486.

Rat (see _Rodentia_).

Rathke, H., on vertebrate embryo, =II=, 119.

Ray, J., plant classification, =I=, 378.

Reasoning, compared with assimilation, =I=, 81–7.

Recapitulation, embryological, =I=, 453.

Regeneration (_see_ Repair).

Rejuvenescence, and sexual fertilization, =I=, 637; =II=, 613.

Remak, R., vertebrate embryo, =II=, 120.

Repair: continuity of, =I=, 216–9;
animal injuries, =I=, 219, 222–4; =II=, 102, 611;
deductive interpretation, =I=, 221–2;
theories of heredity and regenerative phenomena, =I=, 360–1.

Repetition of like parts, =II=, 126.

Reproduction (_see_ Multiplication).

_Reptilia_: growth and expenditure of force, =I=, 142;
sizes of ova and adult, =I=, 144;
longevity of crocodile, =I=, 154;
temperature, =I=, 174;
waste, =I=, 214;
distinctive characters, =I=, 392;
distribution in time, =I=, 409, 412;
vertebral segmentation, =I=, 470;
rudimentary limbs of snakes, =I=, 473;
fertility and development, =I=, 583, 598, 599;
regeneration, =I=, 589;
elongated form, =II=, 15;
supernumerary vertebræ, =II=, 123, 564;
bilateral symmetry, =II=, 203, 204;
Cope on segmentation in extinct, =II=, 225, 226;
activity and muscular colour, =II=, 365;
functional integration, =II=, 375;
outer tissue differentiation, =II=, 387;
Owen on skeleton, =II=, 560.

Resistance of media to locomotion, =II=, 15.

Respiratory System: effect of light, =I=, 31;
organic re-arrangement, =I=, 37;
cutaneous, =I=, 209;
air-cells of lungs, =I=, 254;
embryonic branchiæ of salamander, =I=, 457;
differentiation, =II=, 310–1, 333–8;
physiological integration, =II=, 374–5, 382;
vascular differentiation and integration, =II=, 377.

Retrograde metamorphoses, in animals, =II=, 12.

Retzius, G., superficial nerve-endings, =I=, 666.

Reversed Selection, =I=, 611, 612.

Rhabdospheres, calcareous armour and dynamic element in life, =I=,
119.

Rhizoids, foliar expansions, =II=, 50.

_Rhizopoda_: structure, =I=, 173;
undifferentiated function, =I=, 200;
a primary aggregate, =II=, 86;
symmetry, =II=, 186;
tissue differentiated, =II=, 299, 385;
motion of sarcode, =II=, 356;
symbiosis, =II=, 400.

Rhythm: astronomic and organic, =I=, 499, 557;
law of equilibration, =I=, 520–1;
in multiplication, =II=, 419.

Richeraud, Baron A., definition of life, =I=, 79.

Riley, C. V., on telegony, =I=, 645;
_Termites_, =I=, 680, 681;
pouch of Honey-ants, =I=, 684.

_Rodentia_: incursions, =I=, 399;
American types, =I=, 403;
fertility and development, =I=, 583, 599.

Rivinus, plant classification, =I=, 377.

Rokitansky, on false joints, =I=, 230.

Romanes, G. J.: on “cessation of selection,” =I=, 560–2;
isolation and species differentiation, =I=, 569;
“physiological selection,” =I=, 569–71;
panmixia, =I=, 649, 667;
influence of a previous sire on progeny, =I=, 649.

Röntgen rays, =I=, 121; =II=, 621.

Roots: developed from leaflets, =II=, 34;
physiological differentiation, =II=, 253–5, 270;
nutrition from leaves, =II=, 274;
size and function, =II=, 276.

_Rotiferæ_: latent vitality of desiccated, =I=, 117;
trochopore, =II=, 108, 109;
molluscan relationship, =II=, 115;
fertility and size, =II=, 453, 459.

Roux, W.: “intra-selection,” =I=, 676;
functional adaptation, =II=, 354.

Rudimentary organs: the definition of life and, =I=, 112;
natural selection and eyes of cave fauna, =I=, 309, 612–4, 647–9,
693;
evolution hypothesis, =I=, 472–5, 556;
limbs of whale, =I=, 668–9, 685, 693.

Ruminants, alimentary canal development, =II=, 327–9.

Salamander, embryonic branchiæ, =I=, 457.

_Salmonidæ_, reproduction and growth, =I=, 291–3; =II=, 454.

_Salpidæ_: heterogenesis, =I=, 272, 277;
integration, =I=, 588; =II=, 97.

Sap (_see_ Vascular system).

_Sarcina_: central aggregation, =II=, 24;
fertility, =II=, 440.

Savage, Dr., on “Heredity and Neurosis,” =I=, 313.

_Scenedesmus_, individuation, =II=, 24.

Scent: natural selection and keenness of, =I=, 610;
floral fertilization, =II=, 268–9;
animal protection, =II=, 434.

Schelling, E. W. J. von, definition of life, =I=, 78, 178.

Schleiden, J. M., on individuality, =I=, 245;
on liverworts, =II=, 50, 52;
algal indefiniteness, =II=, 296.

Science, complex revelations of, =I=, 252, 369, 450.

_Scyphomedusæ_, strobilization, =II=, 108.

Sea: changes and movements in, =I=, 83;
life in, lower than terrestrial, =I=, 104;
distribution, =I=, 396, 517;
change of media caused by, =I=, 481;
geologic influence, =I=, 502.

Seals: nail-bearing toes, =I=, 473;
_vibrissæ_, =II=, 317.

Seasons: reproductive periodicity, =I=, 299;
variations of genesis with, =II=, 484–5.

Sedgwick, Adam: on continuity of protoplasm in animals, =I=, 190,
629; =II=, 21;
zoological classification, =I=, 387;
discrimination of species in embryonic stages, =I=, 461;
persistence of ancestral traits, =I=, 463–4;
Archiannelidan segmentation, =II=, 109.

Sedgwick, Wm.: heredity and sex, =I=, 305, 314;
telegonic transmission of hypospadias, =I=, 646.

Seeds: nitrogenous, =I=, 40;
temperature of germinating, =I=, 47; =II=, 615;
vitalism and latent vitality of, =I=, 116–7;
variation in environment, =I=, 327;
natural selection among, =I=, 532.

Segmentation (metameric): special creation hypothesis, =I=, 468–9;
Huxley on number of somites in higher articulates, _ib._;
in annulose animals, =II=, 98–110, 111–5, 601–5;
simulated molluscan, =II=, 116;
in vertebrates, =II=, 125–7, 225–7, 606–7;
in elasmobranchs, =II=, 126.

Segregation: of growth, =I=, 136;
of like units, =I=, 179;
organic repair, =I=, 221;
variation, =I=, 331, 334;
heterogeneity, and definiteness of evolution, =I=, 514–6, 517–8;
morphological development, =II=, 7–9;
physiological units, =II=, 616.

Self-fertilization, animal and vegetal, =I=, 341–4, 353.

Senses, the (_see_ Psychology).

Sex: in Ascidian colonies, =I=, 247;
limitation of heredity by, =I=, 314–6;
correlated traits, =I=, 371–2, 513;
nutrition and determination of, in social insects, =I=, 655–60,
678–84, 686–9;
neural and hæmal traits, =I=, 683;
differentiation of organs, =II=, 303;
castration and growth, =II=, 459;
Julin on “castration parasitaire” in crustaceans, =II=, 493–6;
the object of fertilization, =II=, 613.
(_See also_ Fertilization.)

Sexual Selection (_see_ Natural Selection).

Sharp, D.: on insect somites, =I=, 469;
food habits of _Termites_, =I=, 686–7.

Sheep: contrasted with oxen, =I=, 158, 160;
crossing of English and French breeds, =I=, 625;
nutrition and genesis, =II=, 480.

Sherrington, Prof., on effects of nerve severance, =I=, 349.

Ship-building, interdependence of social functions, =I=, 237–9,
241.

Shipley, A. E.: segmentation of _Microstomida_, =II=, 102;
_Protodrilus_, =II=, 125.

Silica, colloid and crystalloid, =I=, 16.

Silicic acid: properties, =I=, 16;
isomerism, =I=, 59.

Silicon, allotropic, =I=, 4.

Silkworm disease, =I=, 622–3.

Simulation: of homology by analogy, =II=, 14, 485;
of segmented structure by molluscs, =II=, 116.

_Siphonophora_, specialization of component polyps, =II=, 95.

_Sirenia_, simulated fish form, =I=, 485.

Size (_see_ Growth).

Skeleton, vertebrate (see _Vertebrata_).

Skin: respiratory function, =I=, 209;
adaptability, =I=, 228; =II=, 312–4, 387;
transmitted peculiarities, =I=, 306;
Wallace on distribution of sensitiveness, =I=, 646–7;
differentiation, =II=, 215, 217, 304–7;
tegumentary development, =II=, 314–6, 387;
differentiation of sensory organs, =II=, 317–20;
and mucous membrane, =II=, 303–4, 321–2, 389.

“Skin friction,” and locomotion of aquatic animals, =I=, 156.

Skull (see _Vertebrata_).

Sleep, repair favoured by, =I=, 216.

Small-pox, blood changes from, =I=, 221.

Smith, Prof. W., on fertility of _diatomaceæ_, =II=, 440.

Smith, W. P., on telegony in calves and foals, =I=, 645.

Smith, W. W., on habits of Tetramorium, =I=, 660.

Snakes (see _Reptilia_).

“Social organism,” author’s essay on, =I=, 363, 676.

Sociology: environment and degree of life, =I=, 105–6;
functional differentiation, =I=, 204;
division of labour, =I=, 207, 363–4, 367;
functional interdependence, =I=, 237–9, 240–2;
autogenous development of units in colonies, =I=, 364, 367–8;
=II=, 620;
belief in social evolution, =I=, 432;
natural selection, =I=, 553; =II=, 532;
integration and differentiation, =II=, 378–9;
effects of population, =II=, 535–6;
equilibration, =II=, 537.

Soil, dependence of plant evolution on, =II=, 402.

_Solanum jasminoides_, organs of attachment, =II=, 276.

Solar system, autogenous development illustrated by distribution
of forces in, =I=, 366.

Sole, symmetry and location of eyes, =II=, 205.

Soma-plasm, Weismann’s theory of differentiation from germ-plasm,
=I=, 357, 622, 628–30, 633–44.

Somites (_see_ Segmentation).

Special creation: and evolution, =I=, 412, 415, 431;
improbabilities, =I=, 418–9, 430, 439, 554;
inconceivability, =I=, 420, 429, 431, 554;
of individuals and species, =I=, 421–4;
the implication of beneficence, =I=, 425–9;
summary, =I=, 429, 554;
Von Baer’s formula, =I=, 451–6;
vertebrate skeleton, =II=, 551, 556, 565.

Species: adaptation and stability, =I=, 242;
hereditary transmission, =I=, 301–4;
variation in wild and cultivated, =I=, 323–5, 326, 693;
gamogenesis and life of, =I=, 347–9;
physiological units, =I=, 362, 364, 369–71, 458; =II=, 613;
indefiniteness, =I=, 389, 445, 572;
special creation, =I=, 422–4;
instability of homogeneous, and differentiation of, =I=, 509–11,
515, 517–8, 550, 557;
persistence of, =I=, 516, 518; =II=, 10–11;
natural selection and equilibration, =I=, 543–8, 553, 557;
non-adaptive characters, =I=, 565;
morbid products as marks of, =I=, 567;
migration and isolation as causes of differentiation, =I=, 568–9;
increasing multiformity of aggregate, =II=, 396.

Specific gravity, of organisms and environment, =I=, 174, 177.

Spermatozoa}
Sperm-cell } (_see_ Fertilization).

Sphere: tendency of units to form, =I=, 15;
the embryonic form, =I=, 177;
symmetry, =II=, 131.

Spheroid, symmetry, =II=, 132.

Spiders (see _Arachnida_).

Spine (see _Vertebrata_).

Sponge: structure and dynamic element in life, =I=, 119;
multicentral development, =I=, 164;
units and aggregate, =I=, 185;
reproductive tissue, =I=, 283;
integration, =I=, 586; =II=, 90, 383;
physiological differentiation, =II=, 300, 386;
development and genesis, =II=, 463;
analogy from, =II=, 576.

Spontaneous generation: and heterogenesis, =I=, 270;
and evolution, =I=, 696–701, 703.

Stag, horns and correlated structures, =I=, 567, 670, 676–7, 692.

Stamens, and foliar homology, =II=, 44.

Starches: properties, =I=, 11;
transformations, =I=, 66, 68, 69, 70; =II=, 593.

Star-fishes (see _Asteroidea_).

Statoblasts, of _Plumatella_, =I=, 277.

Steenstrup, on “Alternate Generation,” =I=, 592.

Sterility (_see_ Multiplication).

Stickleback: ova, =II=, 454;
_bothriocephalus_ in, =II=, 490.

Stomach (_see_ Alimentary canal).

Stomata, distribution, =II=, 260–1.

Straight line, and evolution hypothesis, =I=, 433.

Strain: compression and tension of, =I=, 151; =II=, 209–12;
relation to mass, =I=, 155–7;
vegetal structure, =II=, 574–88, 592–6;
origin of vertebrate type, =II=, 600.

Strawberry: multiaxial development, =I=, 166;
multiplication, =II=, 441.

Strength, a vital attribute, =I=, 578.

Structure: appliances for generating motion, =I=, 75–7;
biological classification, =I=, 125–7, 129;
size and organic, =I=, 137;
growth and complexity, =I=, 138, 145, 161;
relation to environment, =I=, 172–8, 195–6;
of unicellular organisms, =I=, 181–3;
multicellular, =I=, 183–96;
Hertwig’s classification of tissues, =I=, 189;
continuity of units, =I=, 190–2;
systems of organs, =I=, 192;
division into universal and particular, =I=, 193–4;
general truths, =I=, 194–5;
plant and animal, contrasted, =I=, 195–6;
precedence of function or, =I=, 197, 211;
correlative complexity of function and, =I=, 200, 211;
progressive concomitant differentiation, =I=, 201–4;
physiological units, =I=, 225–6, 362, 364, 369–71; =II=, 613;
social and organic interdependence, =I=, 235–42;
varied by function, =I=, 334, 535; =II=, 217 (_see_ Acquired
Characters);
zoological classification, =I=, 390–2;
equilibration, =I=, 521, 557;
progress of, and genesis, =I=, 590–1; =II=, 462;
co-operation with function, =II=, 3;
evolution and increased, =II=, 4;
retrograde metamorphosis, =II=, 12;
simulated homologies, =II=, 13–14;
earliest organic forms, =II=, 19;
cylindrical vegetal, =II=, 57–62;
permanence and complexity, =II=, 295, 296;
function and epidermic, =II=, 312–4, 387;
and muscular, =II=, 369, 391;
adaptation and equilibration, =II=, 392;
persistence of force and physiological adaptation, =II=, 394;
evolution, =II=, 501–4.
(_See also_ Morphology.)

Struggle, for nutriment among components of an organism, =I=, 562,
676;
for existence (_see_ Natural Selection).

Struthers, Sir J.: on heredity, =I=, 305, 314;
digital variation, =I=, 321;
rudimentary limbs of whale, =I=, 668.

Strychnine, effects of, =I=, 54, 55.

Sturgeon, size of ova and adult, =I=, 144.

Sugars: properties, =I=, 10–11;
transformations, =I=, 38, 40, 66, 69, 70; =II=, 593.

Suicide, hereditary tendency to, =I=, 307.

Sulphur: allotropic, =I=, 4, 59;
organic evolution, =I=, 703.

Sun (_see_ Light).

Survival of the Fittest, the expression, =I=, 530, 610.
(_See_ Natural Selection.)

Swan, vertebræ of neck, =II=, 123.

Swiftness, a vital attribute, =I=, 578.

_Syllis ramosa_, lateral branching, =I=, 166, 361; =II=, 105, 108.

Symbiosis, =II=, 399, 400.

Symmetry (_see_ Morphology).

Syphilis, hereditary transmission, =I=, 623.

Tactual, Perceptiveness, heredity and the distribution of, =I=,
602–8, 633, 665, 666, 672, 692.

_Tænia_ (see _Entozoa_).

Tansley, A. G., =I=, vi; =II=, vi;
adaptation of reproductive activity to conditions in _Algæ_, =I=,
288–9;
shapes of _Caulerpa_, =II=, 22;
stem-thickening in extinct Thallophytes, =II=, 56;
natural selection and leaf-distribution, =II=, 179.

Tape-worm (see _Entozoa_).

Taste, dependent on chemical action, =I=, 54.

Teeth: hereditary transmission, =I=, 306;
suppression of mammalian, =I=, 457;
of uncivilized and civilized, =I=, 541, 693.

Tegumentary organs, origin of, =I=, 314–6.

Telegony, or the Influence of a previous sire on offspring, =I=,
624–7, 644–6, 649–50.

Temperature (_see_ Heat).

Tension (_see_ Strain).

_Termites_: fertility, =I=, 583; =II=, 493;
late development of sexual organs, =I=, 680;
nutrition and differentiation of forms, =I=, 681.

_Tetramorium_, utilization of aphides by, =I=, 660–1.

_Thallophyta_: size, =I=, 138, 139;
low co-ordination of parts, =I=, 164;
pseudo-foliar, =II=, 28;
“transition place,” =II=, 30;
simulation of higher types, =II=, 32;
secondary thickening in extinct species, =II=, 56;
sexual and asexual genesis, =II=, 84.
(See also _Algæ_.)

Tickling, physiology of, =I=, 76.

Tide (_see_ Sea).

Time, as a factor in growth, =II=, 77.

Tissue, Hertwig’s classification, =I=, 189.
(_See_ Physiology.)

Tongue, perceptiveness of tip, =I=, 606–8, 665, 672–3.

Tortoise: contrasted life of dog and, =I=, 103–4;
natural selection and carapace, =I=, 534.

“Transcendental Physiology,” =I=, 176.

Tree, as symbolizing phylogeny, =I=, 428, 452–3.
(_See_ Plants.)

_Trematoda_: agamogenesis, =I=, 277;
parasitism, =I=, 428;
alternate generation, =I=, 592.

Trembley, A., on the polyp, =I=, 223.

Trichinosis, in Germany, =I=, 428.

Trochophore, phyletic relationships shown by, =I=, 447; =II=,
108–9.

_Tubicolæ_: development, =II=, 100;
bilateral symmetry, =II=, 197.

_Tunicata_: gemmation, =I=, 588; =II=, 445;
alternate generation, =I=, 592;
integration, =II=, 93–4;
tertiary aggregation, =II=, 124;
symmetry, =II=, 194–5.

Tunny, size of ova and adult, =I=, 144.

_Turbellaria_: segmentation, =II=, 102;
symbiosis, =II=, 400.

Turnip: chlorophyll in roots, =I=, 209; =II=, 254;
vascular system, =II=, 281, 284, 578, 591, 596.

Twins: similarity of, =I=, 324;
traits of women bearing, =II=, 457.

“Types, persistent,” Huxley on, =I=, 408.

Ulcer, dermal structure, =II=, 306.

Ultimate Reality, incomprehensibility of, =I=, 120.

_Ulva_: cell multiplication, =II=, 26;
outer tissue, =II=, 256.

_Umbelliferæ_: floral symmetry, =II=, 171;
axial and foliar organs, =II=, 541–6.

United States: cases of telegony, =I=, 644–5;
birth-rate, =II=, 520.

Units: differentiation and dissimilarity, =I=, 20;
“protyle,” =I=, 22–3;
shapes in higher types, =I=, 164;
differential assimilation, =I=, 180;
primordial organic, =I=, 181;
morphological composition, =I=, 184–7, 194, 252; =II=, 5, 7–9,
21, 79, 85–6;
segregation and organic repair, =I=, 221–2, 222–6;
chemical, morphological, and physiological, =I=, 225–6; =II=,
612;
stability, =I=, 339;
instability and heterogeneity of organic, =I=, 350;
Darwin’s gemmules, =I=, 356–60, 362, 372;
Weissmann’s germ-plasm (_q. v._) _ib._;
sociological comparison, =I=, 363–8;
specific proclivities in embryogeny, =I=, 458;
phænogamic, =II=, 73, 151;
annulose, =II=, 105;
incident force and homologous, =II=, 159;
morphological summary, =II=, 233.
(_See also_ Physiological Units.)

“Universal Postulate,” =I=, 675.

Unsymmetrical, definition, =II=, 131.

Urea, muscular energy and excretion, =I=, 72.

Van Beneden, P. J., on _Tænia_, =II=, 103.

Variation: digital, =I=, 331;
effects of parental conditions, =I=, 324;
of altered function, =I=, 325, 334, 693;
dissimilarity of initial conditions, =I=, 327–32, 333;
“spontaneous,” =I=, 328, 513, 697; =II=, 529;
persistence of force, =I=, 335;
physiological units, =I=, 348–54, 360, 369, 371–3; =II=, 614–7,
622–3;
Weismann’s germ-plasm theory, =I=, 357–8, 372–3, 671, 677; =II=,
622;
equilibration and vegetal, =I=, 523–5;
Weismann’s panmixia theory, =I=, 561–3, 649, 667–9, 671, 685;
reproductive organs, =I=, 570;
natural selection and concomitant, =I=, 614–21, 653, 664, 674,
692;
and disused organs, =I=, 648, 668;
plus and minus. =I=, 667, 685;
Masters on correlated, in plants, =II=, 298, 621–2;
equilibration of favourable, =II=, 394.

Vascular System: effects of vegeto-alkalies, =I=, 55;
nutrition, =I=, 146, 148;
embryonic development, =I=, 169;
structural traits, =I=, 192, 193;
function, =I=, 199;
of Ascidians. =I=, 202;
functional differentiation and integration, =I=, 205–6;
organic repair, =I=, 217, 221–2;
effect of function, =I=, 229, 234–5, 236;
equilibration, =I=, 535;
community in compound organisms, =I=, 588;
development of vegetal, =II=, 273–5, 279–84, 285–8, 388;
differentiation of, summary, =II=, 288–90, 388;
differentiation of animal, =II=, 339–44;
osseous development, =II=, 347–51;
muscularity, =II=, 364;
muscular colour, =II=, 365–9;
heart-motor apparatus, =II=, 374;
differentiation and integration in animal, =II=, 376–9, 383;
wood formation, =II=, 567–92;
_résumé_ of wood formation, =II=, 592–7.

_Vaucheria_, reproduction, =I=, 279, 289.

Vegetative System, co-ordination of actions in, =I=, 578.

Vegeto-alkalies, physiological effects of, =I=, 54–5.

Velocity, of moving bodies, =II=, 219–20.

_Vertebrata_: size, =I=, 139;
size at birth and maturity, =I=, 144;
axial structure, =I=, 165;
embryonic development and self-mobility, =I=, 175;
functional differentiation, =I=, 206, 591;
reparative power, =I=, 219, 223, 589;
homogenesis universal, =I=, 271;
distinctive traits, =I=, 392; =II=, 35;
distribution in time, =I=, 408;
classificatory value, =I=, 446;
embryonic mammalian respiratory system, =I=, 456;
embryological pre-adaptation, =I=, 461;
evolution and vertebral column, =I=, 470;
rudimentary organs, =I=, 473;
evolution and varied media, =I=, 479–85;
size of head and vertebræ, =I=, 512, 537;
segregation and evolution of vertebræ, =I=, 515;
fertility and development, =I=, 583, 598–9;
Weismann on reproductive cells, =I=, 635;
limb locomotion, =II=, 15;
adaptive segmentation, =II=, 117–23, 125–7, 223, 602, 605–7;
supernumerary vertebræ, =II=, 123;
bilateral symmetry, =II=, 203–6;
internal organic symmetry, =II=, 208;
genesis of rudimentary axis, =II=, 212–6;
natural selection and genesis of structure, =II=, 216, 227;
origin of notochord, =II=, 216–8;
spinal segmentation, =II=, 218–22, 224;
skull development, =II=, 222, 227;
_résumé_ of axis development, =II=, 224;
Cope on author’s theory, =II=, 225–7;
nerve differentiation, =II=, 304;
sensory organs, =II=, 318;
air-chambers, =II=, 334;
osseous differentiation, =II=, 344–55;
activity and muscular colour, =II=, 365–9;
heart-motor apparatus, =II=, 374;
cost of genesis, =II=, 436;
agamogenesis unknown, =II=, 445;
growth and genesis, =II=, 454;
heat expenditure and genesis, =II=, 468–9, 474;
Owen, theory of skeleton, =II=, 548–66;
evolution of vertebræ, =II=, 563–6;
origin of type, =II=, 598–600.

_Vestiges of Creation_, =I=, 491.

Vibrissæ, function of, =I=, 75.

Vitalism, hypothesis examined, =I=, 114–7.

Vittadini, C., on silkworm disease, =I=, 622–3.

Viviparons genesis, =I=, 271, 274–5, 278.

Voice, correlated sexual traits, =I=, 371–2.

Volcano, definition of life and, =I=, 85, 89.

_Volvocineæ_: unicentral development, =I=, 163;
individuality, =I=, 245;
disintegration of genesis, =I=, 276, 587;
spherical aggregation, =II=, 24;
symmetry, =II=, 137, 187;
fertility, =II=, 441.

Vomiting, alimentary canal development, =II=, 328.

_Vorticella_: secondary aggregate, =II=, 90;
symmetry, =II=, 188.

Wallace, A. R.: “The Origin of the Human Races,” =I=, 553;
the expression “Survival of the Fittest,” =I=, 530;
his association of natural with artificial selection, =I=, 609;
co-adaptation in giraffe, =I=, 615;
skin sensitiveness, =I=, 646.

Wasp: co-ordination of instincts in Mason-, =I=, 574, 679–80;
genesis of worker, =I=, 654–7.

Waste, animal, =I=, 69, 213–5, 228;
relation to activity, =I=, 196, 220–1;
in plants, =I=, 213, 220.

Water: properties, =I=, 7, 9;
colloidal affinity for, =I=, 28;
organic change from, =I=, 29;
organic need for, =I=, 147;
proportion in mammalian adult and fœtus, =I=, 154;
motion through, =I=, 156;
organic development and environment, =I=, 173, 177, 479;
terrestrial organisms inhabiting, =I=, 400;
adaptation of organisms to change of media, =I=, 479–85;
vegetal tissue differentiation, =II=, 253;
molecular re-arrangement, =II=, 359;
colloidal contraction, =II=, 361–2.

Water-weed, American, invasion of, =I=, 399.

Watts, Dr., on _The Principles of Biology_, =I=, ix.

Wax, foliar deposit, =II=, 260–1.

Weber, on tactual discriminativeness, =I=, 602.

Weight: relation to environment of organic, =I=, 174, 177;
varying as cube of dimensions, =I=, 151; =II=, 434, 470.

Weismann, Aug.: reproductive tissue in _Medusæ_, =I=, 281;
in _Daphnidæ_, =I=, 290;
his theory of the differentiated germ-plasm and its fundamental
units, =I=, 357, 622–3, 628–30, 633–44, 646; =II=, 618–9,
622;
the alleged differentiation and plant-phenomena, =I=, 359–60;
and regenerative processes, =I=, 360;
false joints, =I=, 362;
implied complexity of determinants, =I=, 370;
theory inadequate to explain correlation of sexual traits, =I=,
372;
and variations in peacock’s tail feather, =I=, 372–3, 695; =II=,
618;
his view of natural selection as sole factor in organic evolution,
=I=, 559;
the doctrine of panmixia, =I=, 561–3, 612, 632, 649, 667–9, 671,
685, 689;
arguments against inheritance of acquired characters, =I=, 612–3,
651–65, 669–71;
blindness of cave-animals, =I=, 613;
current acceptance of his views, =I=, 631, 690;
cannot explain the process of natural selection, =I=, 651;
the degradation of the little toe in man, =I=, 652, 669, 673;
caste gradations of social insects, =I=, 654, 658–65, 670, 675,
678–84, 685;
food-seeking instinct in Amazon ants, =I=, 660, 670;
the co-adaptation of co-operative parts, =I=, 663–4, 670, 674,
675, 676;
tactual discriminativeness, =I=, 665, 672;
intra-selection, =I=, 676–8;
effect of nutrition on fertility of blow-fly, =I=, 678–9.

Whale: weight of brain, =I=, 599;
rudimentary limbs, =I=, 668–9, 685, 693.

Wheat, adaptive variations, =II=, 298.

Whistling, definition of life and, =I=, 112.

White-Cooper, Mr., on inheritance of abnormal vision, =I=, 306.

Willow, nutrition and growth, =I=, 294.

Wilson, E. B.: composition of chromatin, =I=, 260;
separation of segmentation spheres of _Amphioxus_ ovum, =I=, 691.

Wind: and vegetal bilateral symmetry, =II=, 142;
and inner vegetal tissue differentiation, =II=, 275–9, 285, 288,
388;
and proliferation of _Bryophyllum_, =II=, 295;
and vegetal sap movement, =II=, 583, 584, 587;
_résumé_, 592–6.

Wolff, C.: vegetal fructification and nutrition, =I=, 283; =II=,
179–80;
vegetal vascular system, =II=, 283.

Women (_see_ Man).

Wood (_see_ Plants).

Yeast: fermentation, =I=, 38;
fertility, =I=, 581; =II=, 440;
linear aggregation, =I=, 587; =II=, 23.

Zebra marks in horses, =I=, 314.

Zoology, classification, =I=, 124–5, 380–9.

Zoophytes, structural indefiniteness, =I=, 173.

Zoospores, unit-life of, =I=, 185.

Zygote, of conjugating _Algæ_, =I=, 283.

THE END.

FOOTNOTES:

[1] It seems needful here to say, that allusion is made in this paragraph to a proposition respecting the ultimate natures of Evolution and Dissolution, which is contained in an essay on _The Classification of the Sciences_, published in March, 1864. When the opportunity comes, I hope to make the definition there arrived at, the basis of a re-organization of the second part of _First Principles_: giving to that work a higher development, and a greater cohesion, than it at present possesses. [The intention here indicated was duly carried out in 1867.]

[2] Let me here refer those who are interested in this question, to Prof. Huxley’s criticism on the cell-doctrine, published in the _Medico-Chirurgical Review_ in 1853.

A critic who thinks the above statements are “rather misleading” admits that the lowest types of organisms yield them support, saying that “there are certainly masses of protoplasm containing many nuclei, but no trace of cellular structure, in both animals and plants. Such non-cellular masses may exist during development and later become separated up into cells, but there are certain low organisms in which such masses exist in the adult state. They are called by some botanists non-cellular, by others multi-nucleate cells. Clearly the difference lies in the criteria of a cell. There are also some _Protozoa_, and the _Bacteria_, in which no nucleus has certainly been demonstrated. But it is usual to consider the bodies of such organisms as cells nevertheless, and it is supposed that such cells represent a stage of development in which the nucleus has not yet been evolved, though the chemical substance ‘nuclein’ has been formed in some of them.”

Perhaps it will be most correct to say that, excluding the minute, non-nucleated organisms, all the higher organisms--_Metazoa_ and _Metaphyta_--are composed throughout of cells, or of tissues originally cellular, or of materials which have in the course of development been derived from cells. It must, however, be borne in mind that, according to sundry leading biologists, cells in the strict sense are not the immediate products either of the primitive fissions or of subsequent fissions; but that the multiplying so-called cells are nucleated masses of protoplasm which remain connected by strands of protoplasm, and which acquire limiting membranes by a secondary process. So that, in the view of Mr. Adam Sedgwick and others, the substance of an organism is in fact a continuous mass of vacuolated protoplasm.

[3] In further illustration, Mr. Tansley names the fact that in the genus _Caulerpa_ we have extremely complicated forms often of considerable size produced in the same way. The various species simulate very perfectly the members of different groups among the higher plants, such as Horse-tails, Mosses, Cactuses, Conifers and the like.

[4] It may be objected that in _Cladophora_ the separate compartments of the thallus severally contain many nuclei, making it doubtful whether they descend from uni-nucleate cells. If, however, they do not they simply illustrate another form of integration.

[5] The great mass of early ancestral types--plant and animal--consisting of soft tissues, have left no remains whatever, and we have no reason to suppose that those which left remains fell within the direct ancestral lines of any existing forms. Contrariwise, we have reason to suppose that they fell within lines of evolution out of which the lines ending in existing forms diverged. We must therefore infer that the difficulties of affiliation which arise if we contemplate divergent types now existing, would not arise if we had before us all the early intermediate types. The Mammalia differ in sundry respects from all other kinds of Vertebrata--Fishes, Reptiles, Birds; and if the absence of hair, mammæ, and two occipital condyles, in these other vertebrates were taken to imply a fundamental distinction, it might, in the absence of any known fossil links, be inferred that the Mammalia belonged to a separate phylum. But these differences are not held to negative the assumed relationship. Similarly among plants. We must not reject an hypothesis respecting a certain supposed type, because the existing types it must have been akin to present traits which it could not have had. We are justified in assuming, within limits, a hypothetical type, unlike existing types in traits of some importance. Hence results the answer to a criticism passed on the above argument, that it implies relations between the undeveloped and developed forms of the _Jungermanniaceæ_ such as the facts do not show us. This objection is met on remembering that the types in which the supposed transition took place disappeared myriads of years ago.

[6] There is much force in the criticism passed on the above paragraph, and by implication on some preceding paragraphs, that though in plants which tend to produce compound leaves the production is largely dependent on the supply of nutriment, yet the unqualified statement of this relation as a general one, is negatived by the existence of plants which bear only simple leaves, however much high nutrition causes growth. But mostly valid though this objection is, it is probably not universally valid. I am led to say this by what occasionally occurs in flowers. The flowering stem of the Hyacinth is single; but I have seen a cultivated Hyacinth in which one of the flowers had developed into a lateral spike. Still more striking evidence was once supplied to me by Agrimony. All samples of this plant previously seen had single flowering spikes, but some years ago I met with one, extremely luxuriant, in which some flowers of the primitive spike were replaced by lateral spikes; and I am not sure that some of these, again, did not bear lateral spikes. Now if in plants which, in probably millions of cases, have their flowering stems single, excessive nutrition changes certain of their flowers into new spikes, it is a reasonable supposition that in like manner plants which are thought invariably to bear only single leaves, will, under kindred conditions, bear compound leaves.

[7] See _British and Foreign Medico-Chirurgical Review_ for January, 1862.

[8] Schleiden, who chooses to regard as an axis that which Mr. Berkeley, with more obvious truth, calls a mid-rib, says:--“The flat stem of the Liverworts presents many varieties, consisting frequently of one simple layer of thin-walled cells, or it exhibits in its axis the elements of the ordinary stem.” This passage exemplifies the wholly gratuitous hypotheses which men will sometimes espouse, to escape hypotheses they dislike. Schleiden, with the positiveness characteristic of him, asserts the primordial distinction between axial organs and foliar organs. In the higher Archegoniates he sees an undeniable stem. In the lower Archegoniates, clearly allied to them by their fructification, there is no structure having the remotest resemblance to a stem. But to save his hypothesis, Schleiden calls that “a flat stem,” which is obviously a structure in which stem and leaf are not differentiated. He is the more to be blamed for this unphilosophical assumption, since he is merciless in his strictures on the unphilosophical assumptions of other botanists.

[9] To this interpretation it is objected that “the more-developed _Jungermanniaceæ_” do not appear to have arisen from the lower forms of _Jungermanniaceæ_--that is to say, from such lower forms as are now existing. It may, however, be contended that this fact does not exclude the interpretation given; since the higher forms may well have been evolved, not from any of the lower forms we now know, but from lower forms which have become extinct. This, indeed, is the implication of the evolutionary process as pointed out in the note to Chap. I. If then we assume some early type of intermediate structure, the explanation may not improbably hold.

[10] I am indebted to Dr. Hooker for pointing out further facts supporting this view. In his _Flora Antarctica_, he describes the genus _Lessonia_ (see Fig. 37), and especially _L. ovata_, as having a mode of growth simulating that of the dicotyledonous trees, not only in general form but in internal structure. The tall vertical stem thickens as it grows, by the periodical addition of layers to its periphery. That even Thallophytes should thus, under certain conditions, present a transversely-increasing axis, shows that there is nothing absolutely characteristic of Phanerogams in their habit of stem-thickening. Mr. Tansley gives me further verification by the statement that “it is also now certain that members of the _Equisetineæ_ and _Lycopodineæ_, as well as some Ferns which flourished in Carboniferous times, had secondary thickening in their stems quite comparable to that of modern Dicotyledonous trees.”

[11] See note at the end of the chapter.

[12] Since this paragraph was put in type [this refers to the first edition], I have observed that in some varieties of _Cineraria_, as probably in other plants, a single individual furnishes all these forms of leaves--all gradations between unstipulated leaves on long petioles, and leaves that embrace the axis. It may be added that the distribution of these various forms is quite in harmony with the rationale above given.

[13] Since these figures were put on the block, it has occurred to me that the relations would be still clearer, were the primary frond represented as not taking part in these processes of modification, which have been described as giving rise to the erect form; as, indeed, the rooting of its under surface will prevent it from doing in any considerable degree. In such case, each of the Figs. 111 to 117, should have a horizontal rooted frond at its base, homologous with the pro-embryo among Acrogens. This primary frond would then more manifestly stand in the same relation to the rest, as the cotyledon does to the plumule--both by position, and as a supplier of nutriment. Fig. 117_a_, which I am enabled to add, shows that this would complete the interpretation. Of the dicotyledonous series, it is needful to add no further explanation than that the difference in habit of growth, will permit the second frond to root itself as well as the first; and so to become an additional source of nutriment, similarly circumstanced to the first and equal with it.

[14] How the element of time modifies the result, is shown by the familiar fact that crystals rapidly formed are small, and become relatively large when left to form more slowly. If the quantity of molecules contained in a solution is relatively great, so that the mutual polarities of the molecules crowded together in every place throughout the solution are intense, there arises a crystalline aggregation around local axes; whereas, in proportion as the local action of molecules on one another is rendered less intense by their wider dispersion, they become relatively more subordinate to the forces exerted on them by the larger aggregates of molecules that are at greater distances, and thus are left to arrange themselves round fewer axes into larger crystals.

[15] It is objected that these transformations should be much commoner than they are, were they caused solely by the variations of nutrition described. The reply is that they are comparatively rare in uncultivated plants, where such variations are not frequent. The occurrence of them is chiefly among cultivated plants which, being artificially manured, are specially liable to immense accessions of nutriment, caused now by sudden supplies of fertilizing matters, and now by sudden arrival of the roots at such matters already deposited in the soil. It is to these great _changes_ of nutrition, especially apt to take place in gardens, that these monstrosities are ascribed; and it seems to me that they are as frequent as may be expected.

[16] Since this paragraph was published in 1865, much has been learned concerning cell-structure, as is shown in Chapter VI^A of Part I. While some assert that there exist portions of living protoplasm without nuclei, others assert that a nucleus is in every case present, and that where it does not exist in a definite aggregated form it exists in a dispersed form. As remarked in the chapter named, “the evidence is somewhat strained to justify this dogma.” Words are taken in their non-natural senses, if one which connotes an individualized body is applied to the widely-diffused components of such a body; and this perverting of proper meanings leads to obscuration of what may perhaps be an essential truth. As argued in the chapter named (§§ 74_e_, 74_f_), nuclear matter is, as shown by its chemical character, an extremely unstable substance, the molecular changes of which, perpetually going on, initiate shocks, producing changes all around. In the earlier stages of cell-evolution this unstable substance is dispersed throughout the cytoplasm; whereas in the more advanced stages it is gathered together in one mass. If so, instead of saying there is a dispersed nucleus we should say there are the materials of a nucleus not yet integrated.

[17] This statement seems at variance with the figure; but the figure is very inaccurate. Its inaccuracy curiously illustrates the vitiation of evidence. When I saw the drawing on the block, I pointed out to the draughtsman, that he had made the surrounding curves much more obviously related to the contained bodies, than they were in the original (in Dr. Carpenter’s _Foraminifera_); and having looked on while he in great measure remedied this defect, thought no further care was needed. Now, however, on seeing the figure in the printer’s proof, I find that the engraver, swayed by the same supposition as the draughtsman that such a relation was meant to be shown, has made his lines represent it still more decidedly than those of the draughtsman before they were corrected. Thus, vague linear representations, like vague verbal ones, are apt to grow more definite when repeated. Hypothesis warps perceptions as it warps thoughts.

[18] Though the subdivision into chambers of the shell does not correspond to the subdivision into cell-units it may still be held that since in the solitary types the subdivision of the nucleus is followed by formation of new individuals which separate, and since in the compound types the subdivision of the nucleus is followed by growth and formation of new chambers, the compound type must be regarded as an aggregate of the second order.

[19] A critic says the question is “what are the forces internal or external which produce union or separation.” A proximate reply is--degree of nutrition. As in a plant new individuals or rudiments of them are cast off where nutrition is failing, so in a compound animal. The connecting part dwindles if it ceases to carry nutriment.

[20] It has been pointed out that I have here understated the evidence of physiological integration. An instance of it among _Hydrozoa_ is shown in Fig. 151, but by a strange oversight I have forgotten to name the various cases furnished by the _Siphonophora_ in which the individual polypes of a compound aggregate are greatly specialized in adaptation to different functions.

[21] Recently Mr. T. H. Morgan has made elaborate experiments which show that _Planaria Maculata_ may be cut into many pieces from various parts and of various shapes--even a slice out of the side--and each, if not too small, will produce a perfect animal.

[22] Since this was written in 1865 there has come to light evidence more completely to the point than any at that time known. In the subdivision of _Platyhelminthes_ known as _Turbellaria_, there are some, the _Microstomida_ which, by a process of segmentation form “chains of 4, then 8, then 16, and sometimes even 32 individuals.” “Each forms a mouth [lateral] and for some time the chain persists, but the individuals ultimately become sexually matured and then separate.” (Shipley, _Zoology of the Invertebrata_, p. 92.) Here it should be remarked that the lateral mouths enable the members of a string to feed separately, and that nutrition not being interfered with they doubtless gain some advantage by temporary maintenance of their union--probably in creeping.

[23] I find that the reasons for regarding the segment of a _Tænia_ as answering to an individual of the second order of aggregation, are much stronger than I supposed when writing the above. Van Beneden says:--“Le Proglottis (segment) ayant acquis tout son développement, se détache ordinairement de la colonie et continue encore à croître dans l’intestin du même animal; il change même souvent de forme et semble doué d’une nouvelle vie; ses angles s’effacent, tout le corps s’arrondit, et il nage comme une Planaire au milieu des muscosités intestinales.”

[24] Though this was doubtful in 1865 it is no longer doubtful. In an individual _Ctenodrilus monostylus_, which multiplies by dividing and subdividing itself, “parts arise which are destitute of both head and anus and at times consist of only a single segment.” In another species, _C. pardalis_, there is separation into many segments; and each segment before separating forms a budding zone out of which other segments are afterwards produced, completing the animal (Korschelt and Heider, _Embryology_, i, 301–2).

[25] In place of those originally here instanced about which there are disputes, I may give an undoubted one described by McIntosh, the _Syllis ramosa_, a species of chætopod living in hexactinellid sponges from the Arafura Sea, which branches laterally repeatedly so as to extend in all directions through the canals of the sponge. In most cases the buds terminate in oval segments with two long cirri each. But male and female buds were found, provided each with a head, and containing ovaries and testes. Sometimes these sexual buds had become separate from the branched stock.

[26] The name _Annulosa_, once used to embrace the _Annelida_ and _Arthropoda_, has of late ceased to be used. It seems to me better than _Appendiculata_, both as being more obviously descriptive and as being more exclusive.

[27] The fusion of the segments forming the Arthropod head and the extreme changes, or perhaps in some cases disappearances, of their appendages, put great difficulties in the way of identification; so that there are differences of opinion respecting the number of included segments. Prof. MacBride writes:--“It is highly probable that a primary head (præoral lobe or præstomium) has been derived from annelid ancestors, but the secondary fusion of body-segments with this head, in other words the formation of a secondary head, has gone on independently in the different classes of the phylum _Arthropoda_, viz., _Arachnida_, _Crustacea_, and _Tracheata_ (including Insects and Myriapods). Judged by the number of appendages (which gives an inferior limit) the head of a malacostracous Crustacean consists of præstomium and 8 segments; the head of an insect of præstomium and 4 segments; the head of a Myriapod of præstomium and 3 segments; and the head of an Arachnid of præstomium and 3 segments.” Again, the comment of Mr. J. T. Cunningham is:--“According to Claus and most modern authorities there are only 5 segments in the head of an Arthropod, the eyes not counting as appendages; and further it should be noted that the second pair of antennæ are wanting in Insects.”

Of course difference of opinion respecting the number of somites in the head involves difference of opinion respecting the number constituting the entire body, which, in the higher Arthropods, is said by some to be 19 and by others 20. But those who thus differ in detail, agree in regarding all the segments of head and body as homologous, and this is the essential point with which we are here concerned.

[28] Prof. MacBride corrects this statement by saying that “The ctenidia or gills (which in _Mollusca_ generally are represented only by a single pair) are here represented by a large number of pairs; they do not, however, correspond in either number or position to the shell plates.” It may, I think, be contended that if these had any morphological significance, they would not differ in arrangement from the shell plates, and would not be limited to this special type of Mollusc.

[29] Though it is alleged that at a later stage the posterior part of the skull is formed by fusion of divisions which are assumed to represent vertebræ, yet it is admitted that the anterior part of the skull never shows any signs of such division. Moreover in both parts the bones show no trace of primitive segmentation.

[30] See note at the end of the chapter.

[31] A qualifying fact should be named. When the production of vertebral segments has become constitutionally established, so that there is an innate tendency to form them, there arises a liability to form supernumerary ones; and this, from time to time recurring, may lengthen the series, as in the body of a snake or the neck of a swan. This qualification, however, affects equally the hypothesis of an ideal type and the hypothesis of mechanical genesis.

[32] Here and throughout, the word _radial_ is applied equally to the spiral and the whorled structures. These, as being alike on all sides, are similarly distinguished from arrangements that are alike on two sides only.

[33] It should be added that this change of distribution is not due to change in the relative positions of the insertions of the leaves but to their twistings.

[34] We may note that some of these leaves, as those of the Lime, furnish indications of the ratio which exists between the effects of individual circumstances and those of typical tendencies. On the one hand, the leaves borne by these drooping branches of the Lime are with hardly an exception unsymmetrical more or less decidedly, even in positions where the causes of unsymmetry are not in action: a fact showing us the repetition of the type irrespective of the conditions. On the other hand, the degree of deviation from symmetry is extremely variable, even on the same shoot: a fact proving that the circumstances of the individual leaf are influential in modifying its form. But the most striking evidence of this direct modification is afforded by the suckers of the Lime. Growing, as these do, in approximately upright attitudes, the leaves they bear do not stand to one another in the way above described, and the causes of unsymmetry are not in action; and here, though there is a general leaning to the unsymmetrical form, a large proportion of the leaves become quite symmetrical.

[35] It was by an observation on the forms of leaves, that I was first led to the views set forth in the preceding and succeeding chapters on the morphological differentiation of plants and animals. In the year 1851, during a country ramble in which the structures of plants had been a topic of conversation with a friend--Mr. G. H. Lewes--I happened to pick up the leaf of a buttercup, and, drawing it by its foot-stalk through my fingers so as to thrust together its deeply-cleft divisions, observed that its palmate and almost radial form was changed into a bilateral one; and that were the divisions to grow together in this new position, an ordinary bilateral leaf would result. Joining this observation with the familiar fact that leaves, in common with the larger members of plants, habitually turn themselves to the light, it occurred to me that a natural change in the circumstances of the leaf might readily cause such a modification of form as that which I had produced artificially. If, as they often do with plants, soil and climate were greatly to change the habit of the buttercup, making it branched and shrub-like; and if these palmate leaves were thus much overshadowed by one another; would not the inner segments of the leaves grow towards the periphery of the plant where the light was greatest, and so change the palmate form into a more decidedly bilateral form? Immediately I began to look round for evidence of the relation between the forms of leaves and the general characters of the plants they belong to; and soon found some signs of connexion. Certain anomalies, or seeming anomalies, however, prevented me from then pursuing the inquiry much further. But consideration cleared up these difficulties; and the idea afterwards widened into the general doctrine here elaborated. Occupation with other things prevented me from giving expression to this general doctrine until Jan. 1859; when I published an outline of it in the _Medico-Chirugical Review_.

[36] It is objected to the above interpretation that “many flowers of sizes intermediate between the Hollyhock and the Agrimony are radially symmetrical and yet grow sideways. I may mention various _Liliaceæ_, e.g. _Chlorophytum_, _Eucomis_, _Muscari_, _Anthericum_. _Sagittaria_, also, has many of its flowers in this position. Further, if the higher insects alight on flowers in a definite way, as they do, the parts of the flower must bear different relations to the visiting insect, however large, so that flowers unvisited ought all to be zygomorphic.” My reply is that in the sense which here concerns us, the different petals of the Hollyhock-flower do not bear different relations to the visiting insect; since, practically, the upper and lateral petals bear no physical relations at all: in so far as the visiting bee is concerned they are non-existent. The argument implies that change in the form of a flower from the radial to the bilateral is likely to take place only when the contact-relations of the petals to the visiting insect, are such as to make some forms facilitate its action more than others; and the large petals of the Hollyhock cannot facilitate its action at all. In respect of the _Liliaceæ_ instanced, it is needful to inquire whether the structures are such that this alleged cause of bilateral symmetry can come into play.

[37] I had intended here to insert a figure exhibiting these differences; but as the Cow-parsnip does not flower till July, and as I can find no drawing of the umbel which adequately represents its details, I am obliged to take another instance.

[38] It has been pointed out to me that “the extreme development of the corolla so often found in the outer flowers or on the outer side of the outer flowers in closely-packed inflorescences, associated as it often is with disappearance of stamens or carpels or both, is usually put down to specialization of these outer flowers for attractive purposes. Since the whole inflorescence is increased in conspicuousness by such a modification, it is supposed that natural selection favoured those plants which sacrificed a portion of their seed-bearing capacity for the supposed greater advantage of securing more insect visits.” But granting this interpretation, it may still be held that increase of attractiveness due to increase of area must be achieved by florets at the periphery, and that their ability to achieve it depends on their having an outer, unoccupied, space which the inner florets have not; so that, though in a more indirect way, their different development is determined by different exposure to conditions.

[39] One of my critics writes:--“This chapter might of course be enormously extended, not only as in the preceding ones by citation of quite similar cases, but by the introduction of fresh groups of cases.”

[40] Natural selection may have operated in establishing a constitutional tendency to other sudden abridgments. Mr. Tansley alleges that this is a part-cause of the varying distribution of leaves. He says:--“I have myself made some observations on the length of internodes in the Beech, and am satisfied that it follows quite other laws, connected with the suitable disposition of the leaves on the branch. Although I have not had the opportunity of following up this line of work so as in any way to generalize the results, I suspect that ‘indirect equilibration’ is a widespread cause of such variation.”