312 THE EVOLUTION THEORY 



species, whether animal or plant, even every secretion, and indeed 

 every habit, every inherited instinct, is subject to adaptation to the 

 conditions of life. It seems difficult to refuse to admit that this is the 

 natural impression Avhich this study conveys, and it is strengthened 

 as our knowledge increases; that every essential part of a species is 

 not merely regulated hy natural selection, hut is originally proclucecl 

 by it, if not in the species under consideration at the time, then in 

 some ancestral species : and, further, that every part can adjust itself 

 in a high degree to the need for adaptation. It was not without 

 a purpose that I discussed the phenomenon of mimicry so fully, for it, 

 above all others, teaches us how great a power of adaptation the 

 organism possesses, and what insignihcant and small parts may be 

 transformed, in a remarkable degree, in accordance with some actual 

 need. We saw that a butterfly might assume a colouring which 

 diverged entirely from that of its nearest relatives, but which caused 

 it to resemble an immune species of a different family, and thereby 

 protected it more effectively from persecution. Such a case can no 

 more be due to a dominating phyletic force than to a chance and 

 sudden displacement of the state of equilibrium of tlie determinant 

 system ; it can depend only on natural selection, that is, on a sifting 

 out of the diverse variations offered by germinal selection, and the 

 unhampered expression and augmentation of those favoured. 



But it is not only these minute variations, insignificant in relation to 

 the whole structure of the animal, which can be determined by natural 

 selection. The same applies to the phyletic evolution as a whole ; even 

 that is not directed by the assumed internal principle of development. 

 Adaptations, from their very nature, can only depend upon 

 selection, and not upon an internal principle of evolution, since that 

 could take no account whatever of external circumstances, but would 

 cause variations in the organism altogether independently of these. 

 Thus, in considering the origin of any of the larger groups of animals, 

 w^e may exclude a phyletic power as the guide of its evolution as soon 

 as we can prove that all its essential structural relations, as far as 

 they diverge from those of nearly related groups, are adaptations. 

 We may not be able to do this for nearly all of the animal groups, 

 and it will hardly be possible in regard to a single group of plants, 

 because our insight into the biological significance of characters, which 

 means more than the functional significance of the individual parts, 

 and their correlation as parts of a whole, is seldom sufficiently 

 intimate or thorough. But among animals we can do this in regard 

 to some groups ; one of these is the order of whales oi* Cetaceans. 



Cetaceans, as is well known, belong to the Mammalia, that is to 



