102 ORGANIC EVOLUTION — THE FACTORS 



hypothesis that the retrogression of the eyes of cavern- 

 dwelling animals is at all due to increased survival 

 caused by diversion of nutriment to more useful organs, 

 is untenable. Moreover, the case of the eye is quite an 

 exceptional one ; it is an extreme example. Much less 

 can we suppose that the retrogi-ession of other organs is 

 due to economy of nutrition. For instance, as regards 

 the toes of horses ; the modern horse has only one toe 

 on each limb, his ancestors had five. While the transi- 

 tion from five toes to one was occurring, thousands of 

 oenerations lived and died, and tens of thousands of 

 years elapsed. If economy of nutrition, in this case, 

 led to the retrogression of the four toes which are now 

 absent, by influencing the survival rate, what was the 

 amount of nutriment saved for the othei organs by 

 those individuals that varied favourably ? Such an 

 amount only as could have been supplied by an extra 

 mouthful — I had almost said an extra blade — of grass 

 per diem. It is therefore doubly plain that this theory 

 will not boar examination, and that Mr. Spencer is right 

 in ridiculing it. 



As regards the theory that the retrogression of use- 

 less organs is due to the effect that injury to them 

 exercises on the chances of survival, here again the eye 

 is an extreme example. No other organ in the bod}'^ 

 can be thought of as delicate and important which can 

 be as easily injured, and therefore the retrogression of 

 no other organ can be as readily affected by this means. 



To take the example given by Mr. Spencer : " Suppose 

 that in this new habitat the kangaroo had no enemies ; 

 and suppose that, consequently, quickness of hearing 

 not being called for, large ears gave no greater advan- 

 tage than small ones, would an individual with 

 smaller ears tlian usual survive and propagate better 

 than other individuals, in consequence of his ears 

 being less exposed to injury ? Assuredly not." 



