112 ILLUSTRATIONS OF THE ACTION OF [Chap. IV. 



predate how rarely single variations, whether slight or 

 strongly-marked, could be perpetuated. The author 

 takes the case of a pair of animals, producing during 

 their lifetime two hundred offspring, of which, from 

 various causes of destruction, only two on an average 

 survive to pro-create their kind. This is rather an ex- 

 treme estimate for most of the higher animals, but by 

 no means so for many of the lower organisms. He then 

 shows that if a single individual were born, which varied 

 in some manner, giving it twice as good a chance of life 

 as that of the other individuals, yet the chances would 

 be strongly against its survival. Supposing it to survive 

 and to breed, and that half its young inherited the 

 favourable variation; still, as the Eeviewer goes on to 

 show, the young would have only a slightly better 

 chance of surviving and breeding; and this chance 

 would go on decreasing in the succeeding generations. 

 The justice of these remarks cannot, I think, be dis- 

 puted. If, for instance, a bird of some kind could pro- 

 cure its food more easily by having its beak curved, and 

 if one were born with its beak strongly curved, and 

 which consequently flourished, nevertheless there would 

 be a very poor chance of this one individual perpetuating 

 its kind to the exclusion of the common form; but there 

 can hardly be a doubt, judging by what we see taking 

 place under domestication, that this result would follow 

 from the preservation during many generations of a 

 large number of individuals with more or less strongly 

 curved beaks, and from the destruction of a still larger 

 number with the straightest beaks. 



It should not, however, be overlooked that certain 

 rather strongly marked variations, which no one would 

 rank as mere individual differences, frequently recur 



