THE FACTORS OP ORGANIC EVOLUTION. 



imaginable explanation of these save the establishment by 

 inheritance of Lsponta.negi]LS._Yaiiations, such as are known 

 to occur in the human race. 



^ So too, in the third place, with certain alterations in the 



connexions of parts. According to the greater or smaller 



demands made on this 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 



cannot be thus accounted for. It is found, especially at 



r^the extremities, that the relations of tendons to bones and 



j to one another are not always the same. Variations in 



/ their modes of connexion may occasionally prove advan- 



/ tageous, and may thus become established. Here again, 



y then, w r e have a class of structural changes to which 



Mr. Darwin s hypothesis gives us the key, and to which 



there is no other key. 



Once more there are the phenomena of jniimicry^ Per 

 haps in a more striking way than any others, these show 

 how traits w^hich seem inexplicable are explicable as due 

 to the more frequent survival of individuals that have 

 varied in favourable ways. J We are enabled to understand 

 such marvellous simulations as those of the leaf-insect, 

 those of beetles which &quot; resemble glittering dew-drops upon 

 the leaves;&quot; 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 butter 

 fly, rendered so unpalatable to insectivorous birds by its 

 disagreeable taste that they will not catch it, is simulated 

 in its colours and markings by a species which is struc 

 turally quite different so simulated that even a practised 

 entomologist is liable to be deceived : the explanation being 

 that an original slight resemblance, leading to occasional 



