THE FACTORS OF ORGANIC EVOLUTION. 761 



mind that the variety must have existed before Charles the Second's 

 reign, we may assume belongs to something approaching to the hun- 

 dredth generation of these household pets. The relative breadth be- 

 tween the outer surfaces of the zygomatic arches is conspicuously 

 small ; the narrowness of the temporal fossre is also striking ; the 

 zygomata are very slender ; the temporal muscles have left no marks 

 whatever, either by limiting lines or by the character of the surfaces 

 covered ; and the places of attachment for the masseter muscles are 

 very feebly developed. At the Museum of Natural History, among 

 skulls of dogs there is one which, though unnamed, is shown by its 

 small size and by its teeth, to have belonged to one variety or other of 

 lap-dogs, and which has the same traits in an equal degree with the 

 skull just described. Here, then, we have two if not three kinds of 

 dogs which, similarly leading protected and pampered lives, show that 

 in the course of generations the parts concerned in clenching the jaws 

 have dwindled. To what cause must this decrease be ascribed ? Cer- 

 tainly not to artificial selection ; for most of the modifications named 

 make no appreciable external signs : the width across the zygomata could 

 alone be perceived. Neither can natural selection have had anything 

 to do with it ; for even were there any struggle for existence among 

 such dogs, it cannot be contended that any advantage in the struggle 

 could be gained by an individual in which a decrease took place. 

 Economy of nutrition, too, is excluded. Abundantly fed as such dogs 

 are, the constitutional tendency is to find places where excess of ab- 

 sorbed nutriment may be conveniently deposited, rather than to find 

 places where some cutting down of the supplies is practicable. Nor 

 again can there be alleged a possible correlation between these diminu- 

 tions and that shortening of the jaws which has probably resulted 

 from selection ; for in the bull-dog, which has also relatively short 

 jaws, these structures concerned in closing them are unusually large. 

 Thus there remains as the only conceivable cause, the diminution of 

 size which results from diminished use. The dwindling of a little- 

 exercised part has, by inheritance, been made more and more marked 

 in successive generations. 



Difficulties of another class may next be exemplified — those which 

 present themselves when we ask how there can be effected by the 

 selection of favorable variations, such changes of structure as adapt 

 an organism to some useful action in which many different parts co- 

 operate. None can fail to see how a simple part may, in course of 

 generations, be gi'eatly enlarged, if each enlargement furthers, in 

 some decided way, maintenance of the species. It is easy to under- 

 stand, too, how a complex part, as an entire limb, may be increased as 

 a whole by the simultaneous due increase of its co-operative parts ; 

 since if, while it is growing, the channels of supply bring to the limb 

 an unusual quantity of blood, there will naturally result a proportion- 



