12 THE FACTORS OP OEGANIC EVOLUTION. 



are, the constitutional tendency is to find places where 

 excess of absorbed 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 diminutions 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 con 

 ceivable cause, the diminution of size which results from 

 diminished use. The dwindling of a little-exercised part 

 has, by inheritance, &quot;^ecn 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 favourable variations, such 

 changes of structure as adapt an organism to some useful 

 action in which many different parts corpperate. None can 

 fail to see how a simple part may, in course of generations, 

 be greatly enlarged, if each enlargement furthers, in some 

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

 understand, 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 

 proportionately greater size of all its components bones, 

 muscles, arteries, veins, &c. But though in cases like this, 

 the co-operative parts forming some large complex part 

 may be expected to vary together, nothing implies that 

 they necessarily do so ; and we have proof that in various 

 cases, even when closely united, they do not do so. An 

 example is furnished by those blind crabs named in the 

 Origin of Species which inhabit certain dark caves of Ken 

 tucky, and which, though they have lost their eyes, have 



