The supposition of such a phyletic force of transformation (see Appendix I, p.[298]) possesses, in my opinion, the greatest defect that any theory can have,—it does not explain the phenomena. I do not mean to imply that it is incapable of rendering certain subordinate phenomena intelligible, but that it leaves a larger number of facts entirely unexplained. It does not afford any explanation of the purposefulness seen in organisms: and this is just the main problem which the organic world offers for our solution. That species are, from time to time, transformed into new ones might perhaps be understood by means of an internal transforming force, but that they are so changed as to become better adapted to the new conditions under which they have to live, is left entirely unintelligible by this theory. For we certainly cannot accept as an explanation Nägeli’s statement that organisms possess the power of being transformed in an adaptive manner simply by the action of an external stimulus (see Appendix II, p. [300]).
In addition to this fundamental defect, we must also note that there are absolutely no proofs in support of the foundation of this theory, viz. of the existence of an internal transforming force.
Nägeli has very ingeniously worked out his conception of idioplasm, and this conception is certainly an important acquisition and one that will last, although without the special meaning given to it by its author. But is this special meaning anything more than pure hypothesis? Can we say more than this of the ingenious description of the minute molecular structure of the hypothetical basis of life? Could not idioplasm be built up in a manner entirely different from that which Nägeli supposes? And can conclusions drawn from its supposed structure be brought forward to prove anything? The only proof that idioplasm must necessarily change, in the course of time, as the result of its own structure, is to be found in the fact that Nägeli has so constructed it; and no one will doubt that the structure of idioplasm might have been so conceived as to render any transformation from within itself entirely impossible.
But even if it is theoretically possible to imagine that idioplasm possesses such a structure that it changes in a certain manner, as the result of mere growth, we should not be justified in thus assuming the existence of a new and totally unknown principle until it had been proved that known forces are insufficient for the explanation of the observed phenomena.
Can any one assert that this proof has been forthcoming? It has been again and again pointed out that the phyletic development of the vegetable kingdom proceeds with regularity and according to law, as we see in the preponderance and constancy of so-called purely ‘morphological’ characters in plants. The formation of natural groups in the animal and vegetable kingdoms compels us to admit that organic evolution has frequently proceeded for longer or shorter periods along certain developmental lines. But we are not on this account compelled to adopt the supposition of unknown internal forces which have determined such lines of development.
Many years ago I attempted to prove[[177]] that the constitution or physical nature of an organism must exercise a restricting influence upon its capacity for variation. A given species cannot change into any other species, which may be thought of. A beetle could not be transformed into a vertebrate animal: it could not even become a grasshopper or a butterfly; but it could change into a new species of beetle, although only at first into a species of the same genus. Every new species must have been directly continuous with the old one from which it arose, and this fact alone implies that phyletic development must necessarily follow certain lines.
I can fully understand how it is that a botanist has more inclination than a zoologist to take refuge in internal developmental forces. The relation of form to function, the adaptation of the organism to the internal and external conditions of life, is less prominent in plants than in animals; and it is even true that a large amount of observation and ingenuity is often necessary in order to make out any adaptation at all. The temptation to accept the view that everything depends upon internal directing causes is therefore all the greater. Nägeli indeed looks at the subject from the opposite point of view, and considers that the true underlying cause of transformation is in animals obscured by adaptation, but is more apparent in plants[[178]]. Sufficient justification for this opinion cannot, however, be furnished by the fact that in plants many characters have not been as yet explained by adaptation. We should do well to remember the extent to which the number of so-called ‘morphological’ characters in plants has been lessened during the last twenty years. What a flood of light was thrown upon the forms and colours of flowers, so often curious and apparently arbitrary, when Sprengel’s long-neglected discovery was extended and duly appreciated as the result of Darwin’s investigations, and when the subject was further advanced by Hermann Müller’s admirable researches! Even the venation of leaves, which was formerly considered to be entirely without significance, has been shown to possess a high biological value by the ingenious investigations of J. Sachs (see Appendix III, p. [308]). We have not yet reached the limits of investigation, and no reason can be assigned for the belief that we shall not some day receive an explanation of characters which are now unintelligible[[179]].
It is obvious that the zoologist cannot lay too much stress upon the intimate connexion between form and function, a connexion which extends to the minutest details: it is almost impossible to insist too much upon the perfect manner in which adaptation to certain conditions of life is carried out in the animal body. In the animal body we find nothing without a meaning, nothing which might be otherwise; each organ, even each cell or part of a cell is, as it were, tuned for the special part it has to perform in relation to the surroundings.
It is true that we are as yet unable to explain the adaptive character of every structure in any single species, but whenever we succeed in making out the significance of a structure, it always proves to be a fresh example of adaptation. Any one who has attempted to study the structure of a species in detail, and to account for the relation of its parts to the functions of the whole, will be altogether inclined to believe with me that everything depends upon adaptation. There is no part of the body of an individual or of any of its ancestors, not even the minutest and most insignificant part, which has arisen in any other way than under the influence of the conditions of life; and the parts of the body conform to these conditions, as the channel of a river is shaped by the stream which flows over it.
These are indeed only convictions, not real proofs; for we are not yet sufficiently intimately acquainted with any species to be able to recognize the nature and meaning of all the details of its structure, in all their relations: and we are still less able to trace the ancestral history in each case, and to make out the origin of those structures of which the presence in the descendants depends primarily upon heredity. But already a fair advance towards the attainment of inductive proof has been made; for the number of adaptations which have been established is now very large and is increasing every day. If, however, we anticipate the results of future researches, and admit that an organism only consists of adaptations, based upon an ancestral constitution, it is obvious that nothing remains to be explained by a phyletic force, even though the latter be presented to us in the refined form of Nägeli’s self-changing idioplasm.