Chapter II: Preface: To the English Edition

With the appearance of Charles Darwin’s work “On the Origin of Species,” in the year 1858, there commenced a new era in biology. Weary of the philosophical speculations which, at the beginning of this century, had at first been started with moderation but had afterwards been pushed to excess, biologists had entirely let drop all general questions and confined themselves to special investigations. The consideration even of general questions had quite fallen into disuse, and the investigation of mere details had led to a state of intellectual shortsightedness, interest being shown only for that which was immediately in view. Immense numbers of detailed facts were thus accumulated, but they could not possibly be mastered; the intellectual bond which should have bound them together was wanting.

But all this was changed in a short time. At first only single and mostly the younger naturalists fell in with the new theory of development proclaimed by Darwin, but the conviction soon became general that this was the only scientifically justifiable hypothesis of the origin of the organic world.

The materials accumulated in all the provinces of biology now for the first time acquired a deeper meaning and significance; unexpected inter-relations revealed themselves as though spontaneously, and what formerly appeared as unanswerable enigmas now became clear and comprehensible. Since that time what a vast modification has the subject of animal embryology undergone; how full of meaning appear the youngest developmental stages, how important the larvæ; how significant are rudimentary organs; what department of biology has not in some measure become affected by the modifying influence of the new ideas!

But the doctrine of development not only enabled us to understand the facts already existing; it gave at the same time an impetus to the acquisition of unforeseen new ones. If at the present day we glance back at the development of the biological sciences within the last twenty years, we must be astonished both at the enormous array of new facts which have been evoked by the theory of development, and by the immense series of special investigations which have been called forth by this doctrine.

But while the development theory for by far the greater majority of these investigations served as a light which more and more illuminated the darkness of ignorance, there appeared at the same time some other researches in which this doctrine itself became the object of investigation, and which were undertaken with a view to establish it more securely.

To this latter class of work belong the “Studies” in the present volume.

It will perhaps be objected that the theory of descent has already been sufficiently established by Darwin and Wallace. It is true that their newly-discovered principle of selection is of the very greatest importance, since it solves the riddle as to how that which is useful can arise in a purely mechanical way. Nor can the transforming influence of direct action, as upheld by Lamarck, be called in question, although its extent cannot as yet be estimated with any certainty. The _secondary_ modifications which Darwin regards as the consequence of a change in some other organ must also be conceded. But are these three factors actually competent to explain the complete transformation of one species into another? Can they transform more than mere single characters or groups of characters? Can we consider them as the sole causes of the regular phenomena of the development of the races of animals and plants? Is there not perhaps an unknown force underlying these numberless developmental series as the true motor power--a “developmental force” urging species to vary in certain directions and thus calling into existence the chief types and sub-types of the animal and vegetable kingdoms?

At the time these “Studies” first appeared (1875) they had been preceded by a whole series of attempts to introduce into science such an unknown power. The botanists, Nägeli and Askenasy, had designated it the “perfecting principle” or the “fixed direction of variation;” Kolliker as the “law of creation;” the philosophers, Von Hartmann and Huber, as the “law of organic development,” and also “the universal principle of organic nature.”

It was thus not entirely superfluous to test the capabilities of the known factors of transformation. We had here before us a question of the highest importance--a question which entered deeply into all our general notions, not only of the organic world, but of the universe as a whole.

This question--does there exist a special “developmental force”?--obviously cannot be decided by mere speculation; it must also be attempted to approach it by the inductive method.

The five essays in this volume are attempts to arrive, from various sides, somewhat nearer at a solution of the problem indicated.

The first essay on the “Seasonal Dimorphism of Butterflies” is certainly but indirectly connected with the question; it is therein attempted to discover the causes of this remarkable dimorphism, and by this means to indicate at the same time the extent of one of the transforming factors with reference to a definite case. The experiments upon which I base my views are not as numerous as I could desire, and if I were now able to repeat them they would be carried out more exactly than was possible at that time, when an experimental basis had first to be established. In spite of this, the conclusions to which I was led appear to be on the whole correct. That admirable and most conscientious observer of the North American butterflies, Mr. W. H. Edwards, has for many years experimented with American species in a manner similar to that which I employed for European species, and his results, which are published here in Appendix II. to the first essay, contain nothing as far as I can see which is not in harmony with my views. Many new questions suggest themselves, however, and it would be a grateful task if some entomologist would go further into these investigations.

The second essay directly attacks the main problem above indicated. It treats of the “Origin of the Markings of Caterpillars,” and is to some extent a test of the correctness and capabilities of the Darwinian principles; it attempts to trace the differences in form in a definite although small group entirely to known factors.

Why the markings of caterpillars have particularly been chosen for this purpose will appear for two reasons.

The action of Natural Selection, on account of the nature of this agency, can only be exerted on those characters which are of biological importance. As it was to be tested whether, besides Natural Selection and the direct action of external conditions, together with the correlative results of these two factors, there might not lie concealed in the organism some other unknown transforming power, it was desirable to select for the investigation a group of forms which, if not absolutely excluding, nevertheless appeared possibly to restrict, the action of one of the two known factors of transformation, that of Natural Selection; a group of forms consisting essentially of so-called “purely morphological” characters, and not of those the utility of which was obvious, and of which the origin by means of Natural Selection was both possible and probable _ab initio_. Now, although the _colouring_ can readily be seen to be of value to the life of its possessors, this is not the case with the quite independent _markings_ of caterpillars; excepting perhaps those occasional forms of marking which have been regarded as special cases of protective resemblance. The markings of caterpillars must in general be considered as “purely morphological” characters, _i.e._ as characters which we do not know to be of any importance to the life of the species, and which cannot therefore be referred to Natural Selection. The most plausible explanation of these markings might have been that they were to be regarded as ornaments, but this view precludes the possibility of referring them either to Natural Selection or to the influence of direct changes in the environment.

The markings of caterpillars offered also another advantage which cannot be lightly estimated; they precluded from the first any attempt at an explanation by means of Sexual Selection. Although I am strongly convinced of the activity and great importance of this last process of selection, its effects cannot be estimated in any particular case, and the origin of a cycle of forms could never be clearly traced to its various factors, if Sexual Selection had also to be taken into consideration. Thus, we may fairly suppose that many features in the markings of butterflies owe their origin to Sexual Selection, but we are, at least at present, quite in the dark as to how many and which of these characters can be traced to this factor.

An investigation such as that which has been kept in view in this second essay would have been impracticable in the case of butterflies, as well as in the analogous case of the colouring and marking of birds, because it would have always been doubtful whether a character which did not appear to be attributable to any of the other transforming factors, should not be referred to Sexual Selection. It would have been impossible either to exclude or to infer an unknown developmental force, since we should have had to deal with two unknowns which could in no way be kept separate.

We escape this dilemma in the markings of caterpillars, because the latter do not propagate in this state. If the phenomena are not here entirely referable to Natural Selection and the direct action of the environment--if there remains an inexplicable residue, this cannot be referred to Sexual Selection, but to some as yet unknown power.

But it is not only in this respect that caterpillars offer especial advantages. If it is to be attempted to trace transformations in form to the action of the environment, an exact knowledge of this environment is in the first place necessary, _i.e._ a precise acquaintance with the conditions of life under the influence of which the species concerned exist. With respect to caterpillars, our knowledge of the life conditions is certainly by no means as complete as might be supposed, when we consider that hundreds of Lepidopterists have constantly bred and observed them during a most extended period. Much may have been observed, but it has not been thought worthy of publication; much has also been published, but so scattered and disconnected and at the same time of such unequal credibility, that a lifetime would be required to sift and collect it. A comprehensive biology of caterpillars, based on a broad ground, is as yet wanting, although such a labour would be both most interesting and valuable. Nevertheless, we know considerably more of the life of caterpillars than of any other larvæ, and as we are also acquainted with an immense number of species and are able to compare their life and the phenomena of their development, the subject of the markings of caterpillars must from this side also appear as the most favourable for the problem set before us.

To this must be added as a last, though not as the least, valuable circumstance, that we have here preserved to us in the development of the individual a fragment of the history of the species, so that we thus have at hand a means of following the course which the characters to be traced to their causes--the forms of marking--have taken during the lapse of thousands of years.

If with reference to the question as to the precise conditions of life in caterpillars I was frequently driven to my own observations, it was because I found as good as no previous work bearing upon this subject. It was well known generally that many caterpillars were differently marked and coloured when young to what they were when old; in some very striking cases brief notices of this fact are to be found in the works,[1] more especially, of the older writers, and principally in that of the excellent observer Rösel von Rosenhof, the Nuremberg naturalist and miniature painter. In no single case, however, do the available materials suffice when we have to draw conclusions respecting the phyletic development. We distinctly see here how doubtful is the value of those observations which are made, so to speak, at random, _i.e._ without some definite object in view. Many of these observations may be both good and correct, but they are frequently wanting precisely in that which would make them available for scientific purposes. Thus everything had to be established _de novo_, and for this reason the investigations were extended over a considerable number of years, and had to be restricted to a small and as sharply defined a group as possible--a group which was easily surveyed, viz. that of the Hawk-moths or Sphinges.

Since the appearance of the German edition of this work many new observations respecting the markings of caterpillars have been published, such, for example, as those of W. H. Edwards and Fritz Müller. I have, however, made but little use of them here, as I had no intention of giving anything like a _complete_ ontogeny of the markings in all caterpillars: larval markings were with me but means to an end, and I wished only to bring together such a number of facts as were necessary for drawing certain general conclusions. It would indeed be most interesting to extend such observations to other groups of Lepidoptera.

The third essay also, for similar reasons, is based essentially upon the same materials, viz. the Lepidoptera. It is therein attempted to approach the general problem--does there or does there not exist an internal transforming force?--from a quite different and, I may say, opposite point of view. The form-relationships of Lepidoptera in their two chief stages of development, imago and larva, are therein analysed, and by an examination of the respective forms it has been attempted to discover the nature of the causes which have led thereto.

I may be permitted to say that the fact here disclosed of a _different morphological_, with the _same genealogical_ relationship, appears to me to be of decided importance. The agreement of the conclusions following therefrom with the results of the former investigation has, at least in my own mind, removed the last doubts as to the correctness of the latter.

The fourth and shortest essay on the “Transformation of the Axolotl into Amblystoma,” starts primarily with the intention of showing that cases of sudden transformation are no proof of _per saltum_ development. When this essay first appeared the view was still widely entertained that we had here a case proving _per saltum_ development. That this explanation was erroneous is now generally admitted, but I believe that those who suppose that we have here to deal with some quite ordinary phenomenon which requires no explanation, now go too far towards the other extreme. The term “larval reproduction” is an _expression_, but no _explanation_; we have therefore to attempt to find out the true interpretation, but whether the one which I have given is correct must be judged of by others.

These four essays lead up to a fifth and concluding one “On the Mechanical Conception of Nature.” Whilst the results obtained are here summed up, it is attempted to form them into a philosophical conception of Nature and of the Universe. It will be thought by many that this should have been left to professed philosophers, and I readily admit that I made this attempt with some misgiving. Two considerations, however, induced me to express here my own views. The first was that the facts of science are frequently misunderstood, or at any rate not estimated at their true value, by philosophers;[2] the second consideration was, that even certain naturalists and certainly very many non-naturalists, turn distrustfully from the results of science, because they fear that these would infallibly lead to a view of the Universe which is to them unacceptable, viz. the materialistic view. With regard to the former I wished to show that the views of the development of organic Nature inaugurated by Darwin and defended in this work are certainly correctly designated _mechanical_; with reference to the latter I wished to prove that such a mechanical conception of the organic world and of Nature in general, by no means leads merely to one single philosophical conception of Nature, viz. to Materialism, but that on the contrary it rather admits of legitimate development in a quite different manner.

Thus in these last four essays much that appears heterogeneous will be found in close association, viz. scientific details and general philosophical ideas. In truth, however, these are most intimately connected, and the one cannot dispense with the other. As the detailed investigations of the three essays find their highest value in the general considerations of the fourth, and were indeed only possible by constantly keeping this end in view, so the general conclusions could only grow out of the results of the special investigations as out of a solid foundation. Had the new materials here brought together been already known, the reader would certainly have been spared the trouble of going into the details of special scientific research. But as matters stood it was indispensable that the facts should be examined into and established even down to the most trifling details. The essay “On the Origin of the Markings of Caterpillars” especially, had obviously to commence with the sifting and compilation of extensive morphological materials.

AUGUST WEISMANN.

_Freiburg in Baden,
November, 1881._

CONTENTS.

=Part I.=

ON THE SEASONAL DIMORPHISM OF BUTTERFLIES.

I.

_The Origin and Significance of Seasonal Dimorphism_, p. 1.

Historical preliminaries, 1. Does not occur in other orders of insects, 4. Beginning of experimental investigation, 5. Lepidopterous foes, 7. First experiments with _Araschnia Levana_, 10. Experiments with _Pieris Napi_, 13. Discussion of results, 17. Origination of _Prorsa_ from _Levana_, 19. Theoretical considerations, 23. The case of _Papilio Ajax_, 30. Experiments with _Pieris Napi var. Bryoniæ_, 39. The summer generations of seasonally dimorphic butterflies the more variable, 42.

II.

_Seasonal Dimorphism and Climatic Variation_, p. 45.

Distinction between climatic and local varieties, 45. The case of _Euchloe Belia_ and its varieties, 47. The case of _Polyommatus Phlæas_, 49. The case of _Plebeius Agestis_, 50.

III.

_Nature of the Causes producing Climatic Varieties_, p. 52.

Seasonal dimorphism of the same nature as climatic variation, 52. How does climatic change influence the markings of a butterfly? 52. The cause of this to be found in temperature, 54. Part played by the organism itself, 58. Analogous seasonal dimorphism in _Pierinæ_, 60. The part played by sexual selection, 62.

IV.

_Why all Polygoneutic Species are not Seasonally Dimorphic_, p. 63.

Homochronic heredity, 63. Caterpillars, pupæ and eggs of summer and winter generations of seasonally dimorphic butterflies alike, 64. The law of cyclical heredity, 65. Climatic variation of _Pararga Ægeria_, 68. Continuous as distinguished from alternating heredity, 68. Return from dimorphism to monomorphism, 70. Seasonally dimorphic species hibernate as pupæ, 71. Retrogressive disturbance of winter generations, 72. The case of _Plebeius Amyntas_, 75.

V.

_On Alternation of Generations_, p. 80.

Haeckel’s classification of the phenomena, 80. Proposed modification, 81. Derivation of metagenesis from metamorphosis, 82. Primary and secondary metagenesis, 84. Seasonal dimorphism related to heterogenesis, 86. Heterogenesis and adaptation, 89. Differences between seasonal dimorphism and other cases of heterogenesis, 89. The case of _Leptodora Hyalina_, 93.

VI.

_General Conclusions_, p. 100.

Species produced by direct action of environment, 100. The transforming influences of climate, 103. The origin of variability, 107. The influence of isolation, 109. Cyclically acting causes of change produce cyclically recurring changes, 111. Specific constitution an important factor, 112. A “fixed direction of variation,” 114.

_Appendix I._, p. 117.

Experiments with _Araschnia Levana_, 117. Experiments with _Pierinæ_, 122.

_Appendix II._, p. 126.

Experiments with _Papilio Ajax_, 126. Additional experiments with _Pap. Ajax_, 131. Experiments with _Phyciodes Tharos_, 140: with _Grapta Interrogationis_, 149. Remarks on the latter, 152.

_Explanation of the Plates_, p. 159.

=Part II.=

ON THE FINAL CAUSES OF TRANSFORMATION.

=I.=

THE ORIGIN OF THE MARKINGS OF CATERPILLARS.

_Introduction_, p. 161.

I.

_Ontogeny and Morphology of Sphinx-Markings_, p. 177.

The genus _Chærocampa_, 177; _C. Elpenor_, 177; _C. Porcellus_, 184. Results of the development of these species and comparison with other species of the genus, 188. The genus _Deilephila_, 199; _D. Euphorbiæ_, 201; _D. Nicæa_, 207; _D. Dahlii_, 208; _D. Vespertilio_, 209; _D. Galii_, 211; _D. Livornica_, 215; _D. Zygophylli_, 217; _D. Hippophaës_, 218. Summary of facts and conclusions from this genus, 223. The genus _Smerinthus_, 232; _S. Tiliæ_, 233; _S. Populi_, 236; _S. Ocellatus_, 240. Results of the development of these species, 242. The genus _Macroglossa_, 245; _M. Stellatarum_, 245; comparison of this with other species, 253. The genus _Pterogon_, 255; _P. Œnotheræ_, 256; comparison with other species, 256. The genus _Sphinx_, 259; _S. Ligustri_, 259; comparison with other species, 261. The genus _Anceryx_, 264; _A. Pinastri_, 265; comparison with other species, 268.

II.

_Conclusions from Phylogeny_, p. 270.

The Ontogeny of Caterpillars is a much abbreviated but slightly falsified repetition of the Phylogeny, 270. Three laws of development, 274. The backward transference of new characters to younger stages is the result of an innate law of growth, 278. Proof that new characters always originate at the end of the development; the red spots of _S. Tiliæ_, 282.

III.

_Biological Value of Marking in general_, p. 285.

Markings of Caterpillars most favourable to inquiry, 285. Are the Sphinx-markings purely morphological, or have they a biological value? 287.

IV.

_Biological Value of Colour_, p. 289.

General prevalence of protective colouring among caterpillars, 289. Polymorphic adaptive colouring in _C. Elpenor_, _C. Porcellus_, _P. Œnotheræ_, _D. Vespertilio_, _D. Galii_, _D. Livornica_, _D. Hippophaës_, 295. Habit of concealment primary; its causes, 298. Polymorphism does not here depend upon contemporaneous but upon successive double adaptation; displacement of the old by a new adaptation; proof in the cases of _D. Hippophaës_, _D. Galii_, _D. Vespertilio_, _M. Stellatarum_, _C. Elpenor_, and _S. Convolvuli_, 300.

V.

_Biological Value of special Markings_, p. 308.

Four chief forms of marking among _Sphingidæ_, 309. Complete absence of marking among small caterpillars and among those living in obscurity, 310. Longitudinal stripes among grass caterpillars, 312. Oblique striping. Coloured edges are the shadows of leaf ribs, 317. Eye-spots and ring-spots. Definition, 326: Eye-spots not originally signs of distastefulness, 328; they are means of alarm, 329; experiments with birds, 330; possibility of a later change of function in eye-spots, 334. Ring-spots. Are they signs of distastefulness? Are there caterpillars which are edible and which possess bright colours? 335; experiments with lizards, 336. In _D. Galii_, _D. Euphorbiæ_, _D. Dahlii_ and _D. Mauritanica_ the ring-spots are probably signs of distastefulness, 341. In _D. Nicæa_ they are perhaps also means of exciting terror, 342. The primary ring-spot in _D. Hippophaës_ is a means of protection, 344. Subordinate markings. Reticulation, 347. The dorsal spots of _C. Elpenor_ and _C. Porcellus_, 348. The lateral dots of _S. Convolvuli_, 348. Origination of subordinate markings by the blending of inherited but useless markings with new ones, 349.

VI.

_Objections to a Phyletic Vital Force_, p. 352.

Independent origination of ring-spots in species of the genus _Deilephila_, 352. Possible genealogy of this genus, 358. Independent origination of red spots in several species of _Smerinthus_, 360. Functional change in the elements of marking, 365. Colour change in the course of the ontogeny, 367.

VII.

_Phyletic Development of the Markings of the Sphingidæ. Summary and Conclusion_, p. 370.

The oldest _Sphingidæ_ were devoid of marking, 370. Longitudinal stripes the oldest form of marking, 371. Oblique striping, 373. Spot markings, 375. The first and second elements of marking are mutually exclusive, but not the first and third, or the second and third, 377. Results with reference to the origin of markings; picture of their origin and gradual complication, 380. General results; rejection of a phyletic vital force, 389.

=II.=

ON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.

_Introduction_, p. 390.

I.

_Larva and Imago vary in Structure independently of each other_, p. 401.

Dimorphism of one stage only, 402. Independent variability of the stages (heterochronic variability), 403. Constancy and variability are not inherent properties of certain forms of marking, 407. Heterochronic variability is not explained by assuming a phyletic vital force, 410. Rarity of greater variability in pupæ. Greater variability more common among caterpillars than among the imagines. Causes of this phenomenon, 412. Apparent independent variability of the single larval stages. Waves of variability, 416. _Saturnia Carpini_ an instance of _secondary_ variability, 419. Causes of the exact correlation between the larval stages and its absence between the larva and imago, 429.

II.

_Does the Form-relationship of the Larva coincide with that of the Imago?_ p. 432.

Family groups, 432. Families frequently completely congruent, 435. Exception offered by the _Nymphalidæ_, 435. In transitional families the larvæ also show intermediate forms, 441. Genera; almost completely congruent; the Nymphalideous genera can be based on the structure of the larvæ, 444. So also can certain sub-genera, as _Vanessa_, 445. Incongruence in _Pterogon_, 450. Species; incongruence very common; _S. Ocellatus_ and _Populi_, 451. Species of _Deilephila_ show a nearer form-relationship as imagines than as larvæ, 454. Systemy not only the expression of morphological relationship, 455. Varieties; incongruence the rule; seasonal dimorphism; climatic varieties; dimorphism of caterpillars; local varieties of caterpillars, 456. Result of the investigation, 458. Causes of incongruence, 460. A phyletic vital force does not explain the phenomena, 461. This force is superfluous, 464.

III.

_Incongruences in other Orders of Insects_, p. 481.

Hymenoptera. The imagines only possess ordinal characters, 481. Double incongruence: different distance and different group-formation, 483. Diptera, 488. The larvæ form two types depending on different modes of life, 489. The similarity of the grub-like larvæ of Diptera and Hymenoptera depends upon convergence, 494. These data again furnish strong arguments against a phyletic vital force, 496. The tribe _Aphaniptera_, 498. Results furnished by the form-relationship of Diptera and Hymenoptera, 499. Difference between typical and non-typical parts transient, 501.

IV.

_Summary and Conclusion,_ p. 502.

First form of incongruence, 503. Second form of incongruence, 506. General conclusion as to the elimination of a phyletic vital force, 511. Parallelism with the transformation of systems of organs, 513.

_Appendix I._, p. 520.

Additional notes on the Ontogeny, Phylogeny, &c., of Caterpillars. Ontogeny of _Noctua_ larvæ, 520. Additional descriptions of Sphinx-larvæ, 521. Retention of the subdorsal line by ocellated larvæ, 529. Phytophagic variability, 531. Sexual variation in larvæ, 534.

_Appendix II._, p. 536.

_Acræa_ and the _Maracujà_ butterflies as larvæ, pupæ, and imagines, 536.

_Explanation of the Plates_, p. 546.

=Part III.=

ON THE FINAL CAUSES OF TRANSFORMATION (_continued_).

=III.=

THE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.

_Introduction_, p. 555.

_Experiments_, 558. Significance of the facts, 563. The Axolotl rarely or never undergoes metamorphosis in its native country, 565. North American Amblystomas, 570. Does the exceptional transformation depend upon a phyletic advancement of the species? 571. Theoretical bearing of the case, 574. Differences between Axolotl and Amblystoma, 575. These are not correlative results of the suppression of the gills, 578. Explanation by reversion, 581. Cases of degeneration to a lower phyletic stage: Filippi’s sexually mature “_Triton_ larvæ,” 583. Analogous observations on _Triton_ by Jullien and Schreibers, 591. The sterility of the artificially produced Amblystomas tells against the former importance of the transformation, 594. It is not opposed to the hypothesis of reversion, 596. Attempted explanation of the sterility from this point of view, 597. Causes which may have induced reversion in the hypothetical Mexican Amblystomas, 600. Saltness of the water combined with the drying up of the shores by winds, 604. Consequences of the reversion hypothesis, 609; Systematic, 609; an addendum to the “fundamental biogenetic law,” 611; General importance of reversion, 612. _Postscript_; dryness of the air the probable cause of the assumed reversion of the Amblystoma to the Axolotl, 613. _Addendum_, 622.

=IV.=

ON THE MECHANICAL CONCEPTION OF NATURE.

_Introduction_, p. 634.

Results of the three foregoing essays: denial of a phyletic vital force, 634. Application of these results to inductive conclusions with reference to the organic world in general, 636. The assumption of such a force is opposed to the fundamental laws of natural science, 637. The “vital force” of the older natural philosopher, 640. Why was the latter abandoned? Commencement of a mechanical theory of life, 642.

I.

_Are the Principles of the Selection Theory Mechanical?_ p. 645.

Refutation of Von Hartmann’s views, 645. Variability, 646. The assumption of unlimited variability no postulate of the selection theory, 647. The acknowledgment of a fixed and directed variability does not necessitate the assumption of a phyletic vital force, 647. Heredity, 657. Useful modifications do not occur only singly, 657. New characters appearing singly may also acquire predominance, 659. A mechanical theory of heredity is as yet wanting, 665. Haeckel’s “Perigenesis of the Plastidule,” 667. Correlation, 670. The “specific type” depends upon the physiological equilibrium of the parts of the organism, 671. The theoretical principles of the doctrine of selection are thus mechanical, 675. Importance of the physical constitution of the organism in determining the quality of variations, 676. All individual variability depends upon unequal external influences, 677. Deduction of the limitability of variation, 682. Deduction of local forms, 686. Parallelism between the ontogenetic and the phyletic vital force, 687. The two are inseparable, 690.

II.

_Mechanism and Teleology_, p. 694.

Von Baer’s exaction from the theory of selection, 694. Justification of his claim, but the impossibility of the co-operation of a metaphysical principle with the mechanism of Nature, 695. _Per saltum_ development (heterogeneous generation), 698. Weakness of the positive basis of this hypothesis, 699. The latter refuted by the impossibility of the co-operation of “heterogeneous generation” with natural selection, 702. The interruption by a metaphysical principle cannot be reconciled with gradual transformation, 705. The metaphysical (teleological) principle can only be conceived of as the ultimate ground of the mechanism of Nature, 709. Value of this knowledge for the harmonious conception of the Universe, 711. Explanation of the spiritual by the assumption of conscious matter, 714. The theory of selection does not necessarily lead to Materialism, 716.

INDEX p. 719.

STUDIES IN THE THEORY OF DESCENT.