THE FACTOKS OP ORGANIC EVOLUTION. 17 



since any want of adjustment in the blood-supply in this 

 or that set of muscles, would entail incapacity, failure of 

 speed, and loss of life. Moreover the nerves supplying the 

 various sets of muscles would have to be proportionately 

 changed ; as well as the central nervous tracts from which 

 they issued. Can we suppose that all these appropriate 

 changes, too, would be step by step simultaneously made 

 by_fortunate spontaneous variations, occurring along with 

 all the other fortunate spontaneous variations ? Consider 

 ing how immense must be the number of these required 

 changes, added to the changes above enumerated, the 

 chances against any adequate re-adjustments fortuitously 

 arising must be infinity to one. 



If the effects of use and disuse of parts are inheritable, 

 then any change in the fore parts of the giraffe which 

 affects the action of the hind limbs and back, will simul 

 taneously cause, by the greater or less exercise of it, a 

 re-moulding of each component in the hind limbs and 

 back in a way adapted to the new demands; and generation 

 after generation the entire structure of the hind-quarters 

 will be progressively fitted to the changed structure of the 

 fore-quarters : all the appliances for nutrition and innerva- 

 tion being at the same time progressively fitted to both. 

 But in the absence of this inheritance of functionally- 

 produced modifications, there is no seeing how the required 

 re-adjustments can be made. 



Yet a third class of difficulties stands in the way of the 

 belief that the natural selection of useful variations is the 

 sole factor of organic evolution. This class of difficulties, 

 already pointed out in 166 of the Principles of Biology, 

 I cannot more clearly set forth than in the words there 

 used. Hence I may perhaps be excused for here quoting 

 them. 



&quot; Where the life is comparatively simple, or where surrounding circum 

 stances render some one function supremely important, the survival of the 

 fittest may readily bring about the appropriate structural change, without any 



