212 CERTAIN LAWS OF VARIATION. 



appreciable influence upon it. A more potent factor in 

 diminishing variability, or at least in keeping it from 

 increasing, is probably to be found in the greater fer- 

 tility of the most commonly occurring forms of a race. 

 As already mentioned in Chapter III., this phenomenon 

 has thus far been shown to exist only in certain plants, 

 and in a Hydromedusa, but the presumption is that it is 

 of much wider application. Our knowledge of the 

 matter is, however, insufficient to warrant an extended 

 discussion. 



An apparent instance of increase of variability ac- 

 companying cessation of the action of Natural Selection 

 has been furnished by Bumpus,* in the case of the 

 common sparrow. These birds were first introduced 

 from Europe into America in 1850, and they have 

 spread so rapidly, owing to their having abundant food 

 and practically no natural enemies, that the continent is 

 now inundated with them. The Natural Selection, 

 which has kept the birds when in their native habitat 

 more or less subservient to the regulations imposed by 

 competing life, seems in this case to be largely sus- 

 pended. According to Bumpus nearly all the young 

 birds reach maturity; variations in colour and struct- 

 ure, unless most extreme, apparently not being disad- 

 vantageous to their possessor. In order to compare the 

 variability of this American race of sparrows with the 

 native European one, it was found convenient to make 

 measurements of the eggs, rather than of the birds 

 themselves, as they are so much less readily procurable. 

 Eight hundred and sixty-eight American and 868 Eng- 

 lish eggs were compared with respect to length, 



*Biol. Lectures, Wood's Holl, 1896-97, also Science, v. p. 423. 



