Variation and Natural Selection. 75 



that variations in the lengths of the bones in the bat's 

 wing do actually occur in the various individuals of one 

 species ; that the variations are independent ; and that the 

 different species and genera have the character of the wing 

 determined by emphasizing, so to speak, variations in 

 special directions. I make no apology for having treated 

 the matter at some length. Those who do not care for 

 details will judiciously exercise their right of skipping. 



As before mentioned, Mr. Wallace has collected and 

 tabulated other observations on size and length variations. 

 And in addition to such variations, there are the numerous 

 colour-variations that do not admit of being so readily 

 tabulated. Mr. Cockerell tells us that among snail-shells, 

 taking variations of banding alone, he knows of 252 

 varieties of Helix nemoralis and 128 of H. hortensis.* 



That variations do occur under nature is thus un- 

 questionable. And it is clear that all variations necessarily 

 fall under one of three categories. Either they are of 

 advantage to the organism in which they occur ; or they are 

 disadvantageous ; or they are neutral, neither advantageous 

 nor disadvantageous to the animal in its course through life. 



We must next revert to the fact to which attention was 

 drawn in the last chapter, that every species is tending, 

 through natural generation, to increase in numbers. Even 

 in the case of the slow-breeding elephant, the numbers tend 

 to increase threefold in each generation ; for a single pair 

 of elephants give birth to three pairs of young. In many 

 animals the tendency is to increase ten, twenty, or thirty- 

 fold in every generation ; while among fishes, amphibians, 

 and great numbers of the lower organisms, the tendency is 

 to multiply by a hundredfold, a thousandfold, or even in 

 some cases ten thousandfold. But, as before noticed, this 

 is only a tendency. The law of increase is a law of one 

 factor in life's phenomena, the reproductive factor. In any 



* Nature, vol. xli. p. 393. The variation in molluscs is often considerable. 

 In one of the bays in the basement hall of the Natural History Museum is 

 a series showing the variation in size, form, and sculpturing of Paludomus 

 loricatus, which is found in the streams of Ceylon. These varieties have in 

 former times been named as ten distinct species I 



