296 ZOOLOGY. 



the same organs or parts in the offspring. Such a process, 

 at present hypothetical, would afford a better explanation 

 of the evolution of adaptations than any other theory 

 hitherto put forward. 



20. The Two Chief Factors of Evolution. In the 

 above brief outline we have endeavoured to show that 

 there is some valid basis for the conclusion that evolution 

 has been the result not of this single process or the other, 

 but of the interaction of two main factors, one the 

 occurrence of blastogenic mutations which do not directly 

 correspond to external conditions and which have given 

 rise to species and often to genera and to non-adaptive 

 characters generally (metamerism for instance), and the 

 other the direct action of habits and conditions, by me- 

 chanical and functional stimulation, producing adaptive 

 modifications of structure. Every type and every species 

 of animal is the combined result of these two factors : it is 

 neither all adaptation nor all spontaneous mutation, but a 

 combination of the two. 



In some cases, as we have already suggested, a special 

 adaptation may be confined to a single species, but in the 

 majority of cases it is easy to perceive that allied species 

 present differences in adaptive characters which cannot 

 be of importance to the animals in which they occur. For 

 example, if we assume that the function of the antlers of 

 stags is to serve as weapons in fighting, we find that the 

 species of deer are distinguished by differences in the size 

 and branching of the antlers, and in the colour of the 

 coat. It is easy to distinguish the red-deer, the fallow 

 deer, the roe-deer, and the moose or elk. In all these 

 the antlers are different in form and branching, but it is 

 impossible to contend that in each species the special 

 character of the antlers is useful or adapted to the special 

 mode of fighting of that species. In fact Darwin himself 

 was obliged to conclude that the branched antler was 

 more for ornament than use, because a single sharp 

 spike would be a more effective weapon: but there is 

 no adequate basis for the view that the females have any 

 opportunity for selecting branched antlers for their beauty. 



