30 EVOLUTION 01' TO-DA Y. 



This law differs from Darwin's " inherent tendency "" 

 to vary, in that it is supposed to produce definite 

 changes rather than chance variations. , And there 

 are not wanting facts which give support to this 

 view. In the case of domestic races, it is easy to 

 show that the variations are not indefinite, being to 

 a certain extent confined to special directions ; and it 

 is also plain that this definiteness is not wholly ex- 

 plained by external conditions. The pigeons, for 

 instance, vary in some directions, but not in others. 

 They show no tendency to develop the bill of the 

 humming bird or the plumage of the bird of paradise ; 

 nor is there the slightest reason for thinking that 

 they would do so if placed in the same conditions. 

 Moreover, the pigeon varies with the greatest of 

 readiness, while some other animals such as the 

 goose scarcely vary at all. Many instances of 

 this kind might be selected, to show that variations 

 are not indefinite, not entirely dependent on the en- 

 vironment, but are regulated, to some extent, either 

 by natural laws or by the nature of the organism. 

 Still other views of the nature of variations are 

 held. Indeed, the facts of variation are so extremely 

 numerous; they have been collected under such 

 very different circumstances, and by so many 

 hundreds of people ; they are so very varied in 

 their seeming import ; and, above all, so great is 

 our ignorance of the conditions of animals, even 

 when we seem to be well acquainted with them, 

 that it would hardly be possible to invent any law 

 of variation which could not be attested by some 

 facts. 



