NATURAL SELECTION 185 



served. In former editions of this work I sometimes 

 spoke as if this latter alternative had frequently oc- 

 curred. I saw the great importance of individual dif- 

 ferences, and this led me fully to discuss the results of 

 unconscious selection by man, which depends on the 

 preservation of all the more or less valuable individ- 

 uals, and on the destruction of the worst. I saw, also, 

 that the preservation in a state of nature of any occa- 

 sional deviation of structure, such as a monstrosity, 

 would be a rare event; and that, if at first preserved, 

 it would generally be lost by subsequent intercrossing 

 with ordinary individuals. Nevertbeless, until reading an 

 able and valuable article in the "North British Eeview" 

 (1867), I did not appreciate how rarely single variations, 

 whether slight or strongly-marked, could be perpetuated. 

 The author takes the case of a pair of animals, produc- 

 ing during their lifetime two hundred offspring, of which, 

 from various causes of destruction, only two on an aver- 

 age survive to procreate their kind. This is rather an 

 extreme 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 favor 

 able variation; still, as the Reviewer 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 ^L 

 of these remarks cannot, I think, be disputed. If, for 

 instance, a bird of some kind could procure its food 



