Chapter VI: Organic Evolution (3)

Taking a simple animal cell as our starting-point, we have already seen that it performs, in primitive fashion, certain elementary and essential protoplasmic activities, and gives rise to certain products of cell-life. In the metazoa, which are co-ordinated aggregates of animal cells, together with some of their products, there is seen a division of labour and a differentiation of structure among the cells. We see, then, that variation among these related cells has led to differences in size, in form, in transparency, and in function; while the cell-products have been differentiated into those which are of lifelong value, such as bone, cartilage, connective tissue, horn, chitin, etc., together with a variety of colouring matters; those which are of temporary value, such as the digestive secretions, fat, etc.; and those which are valueless or noxious, such as carbonic acid gas and urea, which are excreted as soon as possible. Here are already a number of important and fundamental variations to be accounted for.

Let us notice that, wide as the variations are, they are to a large extent hedged in by physical, chemical, and organic limitations. We have already seen that the size of cells is to a large extent limited, because during growth mass tends to outrun surface; and because, while disruptive changes occur throughout the mass, nutriment and oxygen must be absorbed by the surface. This is a physical limitation. Since the products of cell-life and cell-activity are chemical products, it is clear that they can only be produced under the fixed limitations of chemical combination; and though in organic products these limitations are not so rigid as among inorganic substances, still that there are limitations no chemist is likely to question. The organic limitations are to the varied, but not very numerous, modes of protoplasmic activity.

Probably, even at the threshold of metazoan life, such variations did not affect only individual cells, but rather groups of cells. In other words, the differentiation was at once and primarily a tissue-differentiation. What do we know, however, about the primitive tissue-differentiation of the earliest metazoa? Hardly anything. We may fairly suppose that the first marked difference to appear was that between the outside and the inside. In the formation of an embryo this is the first differentiation we notice. From the beginning of segmentation or, in any case, very early, the outer-layer cells become marked off from the inner-layer cells. The next step was, perhaps, the formation of the mid-layer between the outer and inner. But how further differentiations were effected we really do not know, though we may guess a little. This, perhaps, we may fairly surmise--that fresh differentiations presupposed previous differentiations, and formed the basis of yet further differentiations. Thus calcified cartilage presupposes cartilage, and leads up to the formation of true bone. In all this, however, we are very much in the dark. We can watch, always with fresh wonder, the genesis of tissues in the development of the embryo; but we do not at present know much of the mode of their primitive genesis in the early days of organic evolution: how can we, then, pretend to understand their origins?

If we speculate at all on the matter, we are led to the view that the variations must be primarily due to the differential incidence of mechanical stresses and physical or chemical influences. It may be admitted that this is little more than saying that they are due to some physical cause. Still, this at least may be taken as certain for what it is worth--that the primitive tissue-differentiations are due to physical or chemical influences, direct or indirect, on the protoplasm of the cell. Here is one mode of the origin of variations.

I do not wish to reopen the question whether these variations originate in the germ or in the body. I content myself with indicating the difference, from this standpoint, between the two views. Take, for example, the end-organs of the special senses, which respond explosively to physical influences in ways we shall have to consider more fully in the next chapter. If we hold that variations originating in the body may be transmitted through the germ to the offspring, then we may say that these variations are the direct result of the incidence of the physical or molecular vibrations on the protoplasm. But if we believe, with Professor Weismann, that all variations originate in the germ, then the variations in the end-organs of the special senses, fitting them to be the recipients of special modes of influence, result from physical effects upon the germ of purely fortuitous origin, that is to say, wholly unrelated to the end in view. The rods and cones of the retina are due to purely chance variations, impressed by some chemical or physical causes completely unknown on the germinal protoplasmic substance. Those individuals which did not have these chance variations have been eliminated. It matters not that the rods and cones are believed to have reached their present excellence through many intermediate steps from much simpler beginnings. The fact remains that the origin of all these step-like variations was fortuitous, and not in any way the direct outcome of the physical influences which their products, the rods and cones, have become fitted to receive. I am not at present prepared to accept this theory of the germinal origin of all tissue-variations.

Whether use and disuse are to be regarded as sources of origin of variations is, again, a matter in which there is wide difference of opinion. But if we admit that any variations can take their origin in the body (as distinguished from the germ), then there is no _à priori_ reason for rejecting use and disuse as factors. As such, we are, I think, justified, in the present state of our knowledge, in reckoning them, at all events, provisionally.

It is clear, however, that they are a proximate, not an ultimate, source of origin. I mean that the structures must be there before they can be either strengthened or weakened by use or disuse. They are at most a source of positive or negative variations of existing structures. They cannot be a direct source of origin of superficial variations. Gain or loss of colour; form-variations not correlated with organic variations;--these cannot be directly due to use or disuse. It is in the nervous and muscular systems and the glandular organs that use and disuse are mainly operative. When, however, organs are brought into relation, or fail to be brought into relation, to their appropriate stimuli, we speak of this, too, as use and disuse. We say, for example, that persistent disuse may impair the essential tissues of the recipient end-organs of the special senses, implying that these tissues require to be brought into continued relation to the appropriate stimuli in order that their efficiency be maintained. So, too, we say that the epidermis is thickened by use, meaning that it is brought into relation with certain mechanical stresses. Through correlation, too, the effects of use and disuse may be widespread. Thus increase in the size of a group of muscles may be correlated with increase in the size of the bones to which they are in relation. In fact, so knit together and co-ordinated is the organism into a unity, it is probable that hardly any variation could take place through use or disuse without modifying to some extent the whole organic being.

Once more, let it be clearly remembered that a large and important school of zoologists reject altogether use or disuse as a factor in variation. They believe that those germs are selected through natural selection in which there is an increased tendency to use or disuse of certain organs. In this, however, we are all agreed. The real question is what is the source of origin of this tendency. On the view of germinal origin, we are forced back on unknown physical or chemical influences in no wise related in origin (though, of course, related in result) with the use or disuse to which they give rise.

So far the main distinction between the two biological schools seems to be that the one, placing the origin of variation in the body-tissues, regards the variations as evoked in direct reaction to physical or chemical influences; while the other, placing the origin of variation in the germ, regards the variations as of fortuitous origin.

I do not use the phrase, "of fortuitous origin," as in any sense discrediting the theory. I am not attempting the cheap artifice of damning a view that does not happen to be my own with a phrase or a nickname. And I therefore hasten to point out what variations I do believe to have had a fortuitous origin. The phrase is often misunderstood, and they will serve to explain its meaning.

If the reader will kindly refer to the tables of variations in the bats' wings (Figs. 14-17), he will see that there are a great number of bones which vary in length and vary independently. And if he will also refer to Fig. 18, in which seven species of bats are compared, he will see that the differences arise from the increased length of one set of bones in one species and another set of bones in another species. Now, let us suppose that the long, swallow-like wing of the noctule, a high flyer with rapid wing-strokes, that catches insects in full flight, and the broad wings of the horse-shoe, a low flyer, flapping slowly, and, at any rate, sometimes catching insects on the ground, and covering them with its wings as with a net; let us suppose, I say, that to each species its special form of wing is an advantage. Among thousands of independent variations in the lengths of the bones there would be occasional combinations of variations, giving either increased length or increased breadth to the wing. In the noctule, the former would tend to be selected; in the horse-shoe, the latter. Thus the wing of the noctule would be lengthened, and that of the horse-shoe broadened, through the selection of fortuitous combinations of variations which chanced to be favourable. Now, each individual bone-variation is, we believe, due to some special cause; but the fortunate combination is fortuitous, due to what we term "mere chance."

Darwin believed that chance, in this sense, played a very important part in the origin of those favourable variations for which, as he said, natural selection is constantly and unceasingly on the watch. And there can be little question that Darwin was right.

We must now consider very briefly some of the proximate causes of variations. In most of these cases we cannot hope to unravel the nexus of causation. When a plexus of environing circumstances acts upon a highly organized living animal, the most we can do in the present state of knowledge is to note--we cannot hope to explain--the effects produced.

All readers of Darwin's works know well how insistent he was that the nature of the organism is more important than the nature of the environing conditions. "The organization or constitution of the being which is acted on," he says,[EF] "is generally a much more important element than the nature of the changed conditions in determining the nature of the variation." And, again,[EG] "We are thus driven to conclude that in most cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays when a mass of combustible matter bursts into flame--the nature of the flame depending on the combustible matter, and not on the spark."

Recent investigations have certainly not lessened the force of Darwin's contention. From which there follows the corollary that the vital condition of the organism is a fact of importance. Darwin was led to believe that among domesticated animals and plants good nutritive conditions were favourable to variation. "Of all the causes which induce variability," he says,[EH] "excess of food, whether or not changed in nature, is probably the most powerful." Darwin also held that the male is more variable than the female--a view that has been especially emphasized by Professor W. K. Brooks. Mr. Wallace, as we have already seen, regards the secondary sexual characters of male birds as the direct outcome of superabundant health and vigour. "There is," he says,[EI] "in the adult male a surplus of strength, vitality, and growth-power which is able to expend itself in this way without injury." And Messrs. Geddes and Thomson contend[EJ] that "brilliancy of colour, exuberance of hair and feathers, activity of scent-glands, and even the development of weapons, are in origin and development outcrops of a male as opposed to a female constitution."

There is, I think, much truth in these several views thus brought into apposition. Vigour and vitality, predominant activity and the consequent disruptive changes, with their abundant by-products utilized in luxuriant outgrowths and brilliant colours, are probably important sources of variation. They afford the material for natural selection and sexual selection to deal with. These guide the variations in specific directions. For I am not prepared to press the theory of organic combination so far as to believe that this alone has served to give definiteness to the specific distinctions between secondary sexual characters, though it may have been to some extent a co-operating factor. This, however, is a question apart from that of origin. Superabundant vigour may well, I think, have been a source of _origin_, not only of secondary sexual characters, but of many other forms of variation.

And while these forms of variation may be the special prerogative of the male, we may perhaps see, in superabundant female vigour, a not less important source of developmental and embryonic variations in the offspring. The characteristic selfishness of the male applies his surplus vitality to the adornment of his own person; the characteristic self-sacrifice of the mother applies her surplus vitality to the good of her child. Here we may have the source and origin of those variations in the direction of fosterage and protection which we have seen to have such important and far-reaching consequences in the development of organic life. The storage of yolk in the ovum, the incubation of heavily yolked eggs, the self-sacrificing development in the womb, the elaboration of a supply of food-milk,--all these and other forms of fosterage may well have been the outcome of superabundant female vigour, the advantages of which are thus conferred upon the offspring.

We may now proceed to note, always remembering the paramount importance of the organism, some of the effects produced by changes in the environment.

The most striking and noteworthy feature about the effects of changes of climate and moisture, changes of salinity of the water in aquatic organisms, and changes of food-stuff, is that, when they produce any effect at all, they give rise to _definite_ variations. Only one or two examples of each can here be cited. Mr. Merrifield,[EK] experimenting with moths (_Selenia illunaria_ and _S. illustraria_), finds that the variations of temperature to which the pupa, and apparently also the larva, are subjected tend to produce "very striking differences in the moths." On the whole, cold "has a tendency, operating possibly by retardation, to produce or develop a darker hue in the perfect insect; if so, it may, perhaps, throw some light on the mechanism so often remarked in north-country examples of widely distributed moths." Mr. Cockerell[EL] regards moisture as the determining condition of a certain phase of melanism, especially among Lepidoptera. The same author states that the snail "_Helix nemoralis_ was introduced from Europe into Lexington, Virginia, a few years ago. Under the new conditions it varied more than I have ever known it to do elsewhere, and up to the present date (1890) 125 varieties have been discovered there. Of these, no less than 67 are new, and unknown in Europe, the native country of the species." The effects of the salinity of the water on the brine-shrimp _Artemia_ have already been mentioned. One species with certain characteristics was transformed into another species with other characteristics by gradually altering the saltness of the water. So, too, in the matter of food, the effects of feeding the caterpillars of a Texan species of _Saturnia_ on a new food-plant were so marked that the moths which emerged were reckoned by entomologists as a new species.

The point, I repeat, to be especially noted about these cases and others which might be cited,[EM] is that the variation produced is a definite variation. Very probably it is generally, or perhaps always, produced in the embryonic or larval period of life. In some cases the variation seems to be transmissible, though definite and satisfactory proofs of this are certainly wanting. Still, we may say that if the changed conditions be maintained, the resulting variation will also be maintained. Under these conditions, at least, the variation is a stable one. It is probable that, apart from preferential mating, the varieties thus produced will tend to breed together rather than to be crossed with the parent form or varieties living under different conditions. In this way varieties may sometimes arise by definite and perhaps considerable leaps under the influence of changed conditions. We must not run the adage, _Natura nil facit per saltum_, too hard, nor interpret _saltum_ in too narrow a sense.

It is true, and we may repeat the statement of the fact for the sake of emphasis, that we do not know how or why this or that particular variation should result from this or that change of climate, environment, or food-stuff; nor do we know why certain variations (such as that which produced the ancon breed of sheep) should be stable, while other variations are peculiarly unstable. But in this we are not worse off than we are in the study of inorganic nature. We do not know why calcite should crystallize in any particular one of its numerous varieties of crystalline form; we do not know why some of these are more stable than others. We may be able to point to some of the conditions, but we cannot be said to understand why arragonite should be produced under some circumstances, calcite under others; or why the same constituents should assume the form of augite in some rocks, and hornblende in other rocks. We are hedged in by ignorance; and perhaps one of our chief dangers, becoming with some people a besetting sin, is that of pretending to know more than we are at present in a position to know. Our very analogies by which we endeavour to make clear our meaning may often seem to imply an unwarrantable assumption of knowledge.

In the last chapter I used the term "organic combination," and drew a chemical analogy. I wished to indicate the particularity and the stability of certain variations, and the possibility of new departures through new combinations of variations, the new departure not being necessarily anything like a mean between the combining variations.[EN] I trust that this will not be misunderstood as a new chemico-physical theory of organic forms. I have some fear lest I should be represented as maintaining that a giraffe or a peacock is a definite organic compound, with its proper organic form, in exactly the same way as a rhombohedron of calcite or a rhombic dodecahedron of garnet is a definite chemical compound, with its proper crystalline form. All that the analogy is intended to convey is that variations seem, under certain circumstances, to be definite and stable, and may possibly combine rather than commingle.

_Summary and Conclusion._

It only remains to bring this chapter to a close with a few words of summary and conclusion.

The diversity of animal life must first be grasped. We believe that this diversity is the result of a process or processes of evolution. Evolution is the term applied to continuity of development. It involves adaptation; and adaptation to an unchanging environment may become more and more perfect. But the environment to which organisms are adapted also changes. Where the change is in the direction of complexity, we have elaboration; where it is in the direction of simplicity, we have degeneration. Of these elaboration is the more important. It involves both a tendency to differentiation giving rise to individuality, and a tendency to integration giving rise to association. Continued elaboration is progress; and this is opposed to degeneration.

The factors of evolution fall under two heads--origin and guidance. The origin of variations lies in mechanical stresses, and chemical or physical influences. Whether these act on the body (and are transmitted by inheritance) or only on the germ, is a question which divides biologists into two schools. In the latter case all variations are fortuitous; in the former the development of tissue-variations has been in direct response to the physical or chemical influences. There are, however, in any case fortuitous combinations of variations.

Whether use and disuse are factors of origin is also a debatable point. Those who believe that physical influences on the body are transmissible believe also that the effects of use and disuse are transmissible.

The vital vigour of the organism is a determining condition of importance. The vital vigour of males has favoured the origin of secondary sexual characters; that of females, the fostering and protection of young, and therefore the development in them of vital vigour.

The almost universally admitted factor in guidance is natural selection. But we must be careful not to use it as a mere formula.

Whether sexual selection is also a factor is still a matter of opinion. Without it the specific character and constancy of secondary sexual features are at present unexplained. If inherited use and disuse are admitted as factors in origin, they must also be admitted as important factors in guidance.

Questions of origin and guidance should, so far as is possible, be kept distinct. These terms, however, apply to the origin and guidance of variations. In the origin of species guidance is a factor, no doubt a most important factor. The title of Darwin's great work was, therefore, perfectly legitimate. And those who say that natural selection plays no part in the origin of species are, therefore, undoubtedly in error.

NOTES

[CI] It is beyond the scope of this book to give the _evidences_ of
evolution. Such evidence from embryology, from distribution, and
from palæontology, is now abundant. For palæontological evidence,
see Nicholson's "Manual of Palæontology," 3rd edit., especially
the second volume on "Vertebrates," by R. Lydekker.

[CJ] Weismann, "Essays on Heredity," p. 24.

[CK] Ibid. p. 140.

[CL] Weismann, "Essays on Heredity," p. 90.

[CM] Ibid. p. 292. See also a discussion in _Nature_, in which Mr.
Romanes and Professor Ray Lankester took part, beginning vol. xli.
p. 437.

[CN] Weismann, "Essay on Heredity," p. 140.

[CO] "Origin of Species," p. 110.

[CP] With regard to blind cave-fish, Professor Ray Lankester has
suggested that some selection has been effected. Those animals
whose sight-sensitiveness enabled them to detect a glimmer of
light would escape to the exterior, leaving those with
congenitally weak sight to remain and procreate in the darkness of
the cave.

[CQ] Darwin, "Descent of Man," pt. ii. chap. viii.

[CR] "Darwinism," chap. x.

[CS] "Darwinism," p. 295. Messrs. Geddes and Thomson, "The Evolution of
Sex," p. 28, also contend that "combative energy and sexual beauty
rise _pari passu_ with male katabolism."

[CT] "Darwinism," p. 293.

[CU] Mr. Poulton, who takes a similar line of argument in his "Colours
of Animals," lays special stress upon the production of _white_
(see p. 326).

[CV] See Chapter VIII.

[CW] "Darwinism," p. 172.

[CX] See "Animals and Plants under Domestication," vol. ii. p. 80.

[CY] "Darwinism," p. 332.

[CZ] "The Colour-Sense," by Grant Allen, p. 95.

[DA] That on "The Emotions of Animals" (X.).

[DB] "Darwinism," p. 318.

[DC] Natural History Society of Wisconsin, vol. i. (1889).

[DD] "Darwinism," p. 286.

[DE] On the negative character of disuse, see p. 196.

[DF] Cope, "Origin of the Fittest," p. 374.

[DG] It would appear, from certain passages of his "Darwinism," that
Mr. A. R. Wallace (e.g. p. 139, note) holds or held similar views.
"The genera _Ateles_ and _Colobus_," he says, "are two of the most
purely arboreal types of monkeys, and it is not difficult to
conceive that the constant use of the elongated fingers for
climbing from tree to tree, and catching on to branches while
making great leaps, might require all the nervous energy and
muscular growth to be directed to the fingers, the small thumb
remaining useless." I should also have quoted Mr. Wallace's
account of the twisting round of the eyes of flat-fishes--where he
says that the constant repetition of the effort of twisting the
eye towards the upper side of the head, when the bony structure is
still soft and flexible, causes the eye gradually to move round
the head till it comes to the upper side--had he not subsequently
disclaimed this explanation (see _Nature_, vol. xl. p. 619). It is
possible that Mr. Wallace, notwithstanding the words "constant
use" in the passage I have quoted, merely intends to imply that
the elongated fingers are of advantage in climbing, and are thus
subject to natural selection, the thumb diminishing through
economy of growth.

[DH] I find, on rereading one of his articles, that I have here
unwittingly adopted one of Mr. Romance's arguments (see _Nature_,
vol. xxxvi. p. 406). The instance Mr. Romanes cites is the curious
habit of dogs turning round before they lie down.

[DI] Mr. Darwin, while contending that the modifications need not all
have been simultaneous, says, "Although natural selection would
thus tend to give the male elk its present structure, yet it is
probable that the inherited effects of use, and of the mutual
action of part on part, have been equally or more important"
("Animals and Plants under Domestication," vol. ii. p. 328).

[DJ] _Midland Naturalist_, November, 1889.

[DK] See _ante_, p. 52.

[DL] _Nature_, vol. xli. p. 511.

[DM] "Animals and Plants under Domestication," vol. ii. p. 291.

[DN] In the third chapter we saw that in such cases not only are there
an enormous number of ova produced, but that (e.g. in aurelia and
the liver-fluke) each ovum produces, through the intervention of
asexual multiplication, many individuals.

[DO] Cope, "Origin of the Fittest," pp. 226, 125, and 297.

[DP] "Animals and Plants under Domestication," vol. ii. p. 313.

[DQ] Ibid. p. 56.

[DR] _Nature_, vol. xxxvi. p. 592.

[DS] Quoted from "Medical Notes and Reflections," 1855, p. 267, by
Darwin, "Animals and Plants under Domestication," vol. i. p. 446.

[DT] Darwin, "Animals and Plants under Domestication," vol. i. p. 465.

[DU] "Natural Inheritance," p. 12.

[DV] Darwin, "Animals and Plants under Domestication," vol. ii. p. 70.

[DW] "Organic Evolution," Mr. Cunningham's translation, p. 76.

[DX] Darwin, "Animals and Plants under Domestication," vol. i. p. 104.

[DY] Similarly, from a chance sport of a one-eared rabbit, Anderson
formed a breed which steadily produced one-eared rabbits ("Animals
and Plants under Domestication," vol. i. p. 456). This is an
example of asymmetrical variation. Variations are generally, but
not always, symmetrical. Superficial colour-variations are
sometimes asymmetrical. Gasteropod molluscs are nearly always
asymmetrically developed. Among insects, _Anisognathus_ affords an
example of the asymmetrical development of the mandible. Our
right-handedness is a mark of asymmetry.

[DZ] "Natural Inheritance," p. 32.

[EA] See "Animals and Plants under Domestication," vol. ii. p. 40, from
which illustrations are taken.

[EB] "Evolution and Disease," p. 169.

[EC] "Animals and Plants under Domestication," vol. ii. p. 8.

[ED] "Darwinism," p. 107.

[EE] Darwin, "Animals and Plants under Domestication," vol. ii. pp. 17,
18.

[EF] "Animals and Plants under Domestication," vol. ii. p. 201.

[EG] Ibid. p. 282. The phenomena of the seasonal dimorphism of
butterflies and moths show that changes of temperature (and
perhaps moisture, etc.) determine very striking differences in
these insects.

[EH] "Animals and Plants under Domestication," vol. ii. p. 244.

[EI] "Darwinism," p. 293.

[EJ] "Evolution of Sex," p. 22.

[EK] "Incidental Observations in Pedigree Moth-breeding," F.
Merrifield. Transactions Entomological Society, 1889, pt. i. p.
79, _et seq._

[EL] _Nature_, vol. xli. p. 393.

[EM] See Professor Meldola's edition of Professor Weismann's "Studies
in the Theory of Descent," and Mr. Cunningham's translation of
Professor Eimer's "Organic Evolution."

[EN] See Darwin, "Animals and Plants under Domestication," vol. ii. p.
252.