NATUBAL SELECTION 79 



case the variation is lost by mingling with the com- 

 mon stock. 



Mr. Darwin says that very " rarely single varia- 

 tions, whether slight or strongly marked, could be 

 perpetuated." 



He says: "If, for instance, a bird of some kind 

 could procure its food more easily by having its beak 

 curved, and if one were born with its beak strongly 

 curved, and which consequently flourished, neverthe- 

 less, 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 domestica- 

 tion, that this result would follow from the preserva- 

 tion during many generations of a large number of 

 individuals with more or less curved beaks, and from 

 the destruction of a still larger number with the 

 straightest beaks."* 



Here he admits that even a favorable variation in 

 a single individual is not at all likely to be propa- 

 gated, but he attempts to bridge the difficulty by 

 assuming that the same favorable variation may take 

 place at the same time in a large number of individ- 

 uals, so that the favorable variation, in spite of ming- 

 ling with all other variations, can be propagated. 



I think that this assumption is destitute of the sup- 

 port of facts. The fact is that variations take place 

 in all conceivable directions, and, as Darwin repeat- 

 edly urges, they are generally very slight. Varia- 

 tions in one direction are, on an average, probably 

 equal to those in another, and, consequently, when 

 all the variations are mingled, as they are in a state 

 of nature, the form of the species remains substan- 

 tially constant. 



Besides, this theory, if true, would account for 



* Origin of Species, 5th Edition, p. 94. 



