It is certainly a strange conclusion this, at which Nägeli finally arrives, for, after strenuously combating the idea that the external factors of climate and of food have influence in producing new species, he does not hesitate to ascribe all sorts of imaginary influences to other external causes. The apparent contradiction is due, perhaps, to the fact that his experience with actual species led him to deny that the direct action of the environment produces permanent changes, while in theory he saw the necessity of adding to his perfecting principle some other factor to explain the adaptations of the new forms produced by inner causes. Nägeli seems to have felt strongly the impossibility of explaining the process of evolution and of adaptation as the outcome of the selection of chance variations, now in this direction, now in that. He seems to have felt that there must be something within the organism that is driving it ever upward, and he attempts to avoid the teleological element, which such a conception is almost certain to introduce, by postulating the inheritance of the effects of long-continued action of the environment, in so far as certain factors in the environment produce a response in the organism. Nevertheless, this combination is not one that is likely to commend itself, aside from the fact that the assumptions have no evidence to support them. Despite Nägeli’s protest that his principles are purely physical, and that there is nothing mystical in his point of view, it must be admitted that his conception, as a whole, is so vague and difficult in its application that it probably deserves the neglect which it generally receives.
Nägeli’s wide experience with living plants convinced him that there is something in the organism over and beyond the influence of the external world that causes organisms to change; and we cannot afford, I think, to despise his judgment on this point, although we need not follow him to the length of supposing that this internal influence is a “force” driving the organism forward in the direction of ever greater complexity. A more moderate estimate would be that the organism often changes through influences that appear to us to be internal, and while some of the changes are merely fluctuating or chance variations, there are others that appear to be more limited in number, but perfectly definite and permanent in character. It is the latter, which, I believe, we can safely accredit to internal factors, and which may be compared to Nägeli’s internal causes, but this is far from assuming that these changes are in the direction of greater completeness or perfection, or that evolution would take place independently of the action of external agencies.
CHAPTER X
THE ORIGIN OF THE DIFFERENT KINDS OF ADAPTATIONS
In the present chapter we may first consider, from the point of view of discontinuous variations as contrasted with the theory of the selection of individual variations, the structural adaptations of animals and plants, i.e. those cases in which the organism has a definite form that adapts it to live in a particular environment. In the second place, we may consider those adaptations that are the result of the adjustment of each individual to its surroundings. In subsequent chapters the adaptations connected with the responses of the nervous system and with the process of sexual reproduction will be considered.
It should be stated here, at the outset, that the term mutation will be used in the following chapters in a very general way, and it is not intended that the word shall convey only the idea which De Vries attaches to it; it is used rather as synonymous with discontinuous and also definite variation of all kinds. The term will be used to include “the single variations” of Darwin, “sports,” and even orthogenic variation, if this has been definite or discontinuous.
Form and Symmetry
Almost without exception, animals and plants have definite and characteristic forms. In other words, they are not amorphous masses of substance. The members of each species conform, more or less, to a sort of ideal type. Our first problem is to examine in what sense the form itself may be looked upon as an adaptation to the surroundings.
It is a well-recognized fact that the forms of many animals appear to stand in a definite relation to the environment. For instance, animals that move in definite directions in relation to their structure have the anterior and the posterior ends quite different, and it is evident that these ends stand in quite different relations to surrounding objects; while, on the other hand, the two sides of the body which are, as a rule, subjected to the same influences are nearly exactly alike. The dorsal and the ventral surfaces of the body are generally exposed to very different external conditions, and are quite different in structure.
The relation is so obvious in most cases that it might lead one quite readily to conclude that the form of the animal had been moulded by its surroundings. Yet this first impression probably gives an entirely wrong conception of how such a relation has been acquired. Before we attempt to discuss this question, let us examine some typical examples.