XXV.] THE OPERATION OF NATURAL SELECTION. 339 



increase of power to fight the battle of life, and consequently 

 an increased chance of surviving and leaving offspring. This, 

 however, is only an average result, not a uniform or constant 

 one. For instance, if an animal becomes internally or externally Excep- 

 parasitic on other animals, it wUl have little or no occasion for ^^°'^^- 

 acute senses or great motor power ; its organs of sense and Retro- 

 motion will consequently degenerate, and the whole organism, ^^'^^^ 

 taken altogether, will undergo a retrograde change ; while the 

 great facility of obtaining abundant nourishment which its 

 parasitic life afl'ords, will cause the variation to be a favourable 

 one, and it will be perpetuated. Such has no doubt been the Suctorial 

 origin of the " suctorial parasites " on fishes and whales, into P^^'^'^^ites. 

 which many freely swimming crustacean larvae are metamor- 

 phosed. But such cases are distinctly exceptional. As a rule, 

 natural selection will preserve only those variations which con- 

 stitute an advance in organization, and the rest will perish. 



Here another question arises, which Darwin has not seen, but 

 which H. Spencer has seen and satisfactorily answered.^ The 

 ability of an individual, or of a species, to survive in the battle 

 of life no doubt depends on its organization ; and the chances Chance of 

 are, on the whole, in favour of the highest organization. But offsI|J;5„ 

 the probability of its not only surviving but leaving offspring, partly 

 does not depend on tliis exclusively ; it depends partly on its "■.' 

 organization, but partly on its fecundity. Other things being fecundity. 

 equal, the highest organization will have the best chance. But, 

 other things being equal, the greatest fecundity will have the 

 best chance. The chance that any variety will have of being 

 preserved by natural selection will be in a ratio compounded of 

 its organization and its fecundity. 



The advantage that fecundity gives to a species may be seen Rabbit 

 by comparing the rabbit with the hare. The hare would pro- ^ 

 bably be by this time extinct in the cultivated parts of our 

 country, were it not preserved, while the rabbit has no difii- 

 culty in maintaining its position; a difference which may be 

 partly due to the burrowing habits of the latter, but much more, 

 I think, to its great fecundity. 



To resume the argument. Fecundity and high organization High 

 do not in general accompany each other, but the reverse. The ^''S'*^^^" 



1 "What follows is in substance taken from Spencer's Principles of 

 Biology, Part VI., especially chapters ix. and xi. The algebraic statement 

 of the reasoning is my own. 



