INTRODUCTION 11 



problem how it is that the same change of conditions produces 

 different effects in different cases. It is possible, however, 

 to investigate the subject by observation and experiment. 

 The selection theory does not explain the origin of specific 

 characters, it only supposes that the variations when they 

 appeared were selected because of some advantage they con- 

 ferred. We are not compelled to accept this view because 

 we cannot explain the origin ourselves. We also suppose 

 that the variations occurred, and being either indifferent, or 

 not sufficiently harmful to cause the death of the individual, 

 were transmitted to the progeny. They would thus become 

 common to the species, but could not be transferred to other 

 species, because species do not breed together. The first step 

 in the differentiation of species, therefore, is the existence of 

 barriers to interbreeding. The view that specific characters 

 are of some use or significance in the struggle for existence is 

 in a vast number of cases unsupported by any evidence. 



Having thus indicated the reasons for rejecting the con- 

 clusion that all distinguishing characters are adaptive or 

 advantageous, we may proceed to consider the origin of 

 adaptations. 



It may be truly said that no animal is without adapta- 

 tions ; it must be provided with some means by which the 

 essentials of life are secured, but these means may be exceed- 

 ingly simple or exceedingly complex. But there is another 

 idea implied in the conception of adaptation, the idea of 

 unity in diversity, of parts essentially similar being modified 

 in different animals for different purposes, being adapted in 

 many cases for purposes quite different from that which they 

 originally served, as in the case of the fore-leg becoming in 

 birds the wing. It is possible to trace such modifications 

 and more or less disguised homologies without any reference 

 to the doctrine of evolution. The principle of descent with 

 modification gives the explanation of the phenomenon. The 



