XXIV.] NATUEAL SELECTION. 319 



of the lowest organisms, in whicli nerves cannot be detected, 

 are known to be sensitive to light, it does not appear im- 

 probable that certain elements in their tissues or sarcode 

 should have become aggregated and developed into nerves 

 endowed with special sensibility to its action." ^ This is 

 in accordance with all that we know of the process of 

 development; and I believe as firmly as Darwin can do 

 that the eyes of the highest animals, like everything else 

 in the organic creation, have been brought to their present 

 perfection by adding together a number of small improve- 

 ments, continu.ed through an immense number of gene- 

 rations. But it does not follow that these improvements, 

 even though almost infinitely small, can be due to 

 natural selection among spontaneous variations. The 

 higher the organization, whether of an entire organism or 

 of a single organ, the greater is the number of the parts 

 that co-operate, and the more perfect is their co-operation ; 

 and consequently, the more necessity there is for corre- 

 sponding variations to take place in all the co-operating 

 parts at once, and the more useless will be any variation 

 whatever, unless it is accompanied by corresponding Natural 

 variations in the co-operating parts ; while it is obvious inlp^phv 

 that the greater the number of variations which are needed able to the 

 in order to effect an improvement, the less will be the organiza- 

 probability of their all occurring at once. It is no reply *^™- 

 to this to say, what is no doubt abstractly true, that what- 

 ever is possible becomes probable, if only time enough 

 be allowed. There are improbabilities so great that the Improba- 

 common sense of mankind treats them as impossibilities, equa"! to 

 It is not, for instance, in the strictest sense of the word, impossi- 

 impossible that a poem or a mathematical proposition 

 should be obtained by the process of shaking letters out 

 of a box ; but it is improbable to a degree that cannot be 

 distinguished from impossibility ; and the improbability of 

 obtaining an improvement in an organ by means of several 

 spontaneous variations all occurring together is an impro- 

 bability of the same kind.^ It is, of course, out of the 

 question to find numerical data on such subjects, but the 



^ Origin of Species, p. 215. ^ See Note at end of chapter. 



