THE FACTORS OF ORGANIC EVOLUTION. 759 



or that limb, the muscles moving it may be augmented or diminished 

 in bulk ; and, if there is inheritance of changes so wrought, the limb 

 may, in course of generations, be rendered larger or smaller. But 

 changes in the arrangements or attachments of muscles can not be 

 thus accounted for. It is found, especially at the extremities, that the 

 relations of tendons to bones and to one another are not always the 

 same. Variations in their modes of connexion may occasionally prove 

 advantageous, and may thus become established. Here again, then, 

 we have a class of structural changes to which Mr. Darwin's hypothe- 

 sis gives us the key, and to which there is no other key. 



Once more there are the phenomena of mimicry. Perhaps in a 

 more striking way than any others, these show how traits which seem 

 inexplicable are explicable as due to the more frequent survival of in- 

 dividuals that have varied in favorable ways. We are enabled to 

 understand such marvellous simulations as those of the leaf-insect, 

 those of beetles which ''resemble glittering dew-drops upon the 

 leaves ; " those of caterpillars, which, when asleep, stretch themselves 

 out so as to look like twigs. And we are shown how there have arisen 

 still more astonishing imitations — those of one insect by another. As 

 Mr. Bates has proved, there are cases in which a species of butterfly, 

 rendered so unpalatable to insectivorous birds by its disagreeable taste 

 that they will not catch it, is simulated in its colors and markings by 

 a species which is structurally quite different — so simulated that even 

 a practised entomologist is liable to be deceived : the explanation be- 

 ing that an original slight resemblance, leading to occasional mistakes 

 on the part of birds, was increased generation after generation by the 

 more frequent escape of the most-like individuals, until the likeness 

 became thus great. 



But now, recognizing in full this process brought into clear view 

 by Mr. Darwin, and traced out by him with so much care and skill, 

 can we conclude that, taken alone, it accounts for organic evolution ? 

 Has the natural selection of favorable variations been the sole factor ? 

 On critically examining the evidence, we shall find reason to think 

 that it by no means explains all that has to be explained. Omitting 

 for the present any consideration of a factor which may be distin- 

 guished as primordial, it may be contended that the above-named 

 factor alleged by Dr. Erasmus Darwin and by Lamarck, must be rec- 

 ognized as a co-operator. Utterly inadequate to explain the major 

 part of the' facts as is the hypothesis of the inheritance of functionally- 

 produced modifications, yet there is a minor part of the facts, very ex- 

 tensive though less, which must be ascribed to this cause. 



When discussing the question more than twenty years ago {Princi- 

 2jles of Biolorjy, § 166), I instanced the decreased size of the jaws in 

 the civilized races of mankind, as a change not accounted for by the 

 natural selection of favorable variations ; since no one of the decre- 



