EVOLUTION OF THE COLORS OF BIRDS. 103 



THE NATURE OF SPECIES. 



Throughout all the foregoing consideration of the 

 various theories and speculations regarding the factors 

 of organic evolution, it will be noticed that little has 

 been said about the nature and origin of species. This 

 has been left for the conclusion, as the final problem for 

 evolution to solve. In the first place, a few words on 

 the term species will be necessary. In its broadest or 

 logical interpretation, any set of individuals having 

 some characteristic or group of characteristics in com- 

 mon, constitutes a species. Species may be determined 

 upon an artificial or upon a natural basis. For example, 

 the human race might be divided into species depend- 

 ing upon the occupation pursued, when we would have 

 a species of hunters, a species of mechanics, one of 

 doctors, another of lawyers, and so on, ad inftnitum. 

 This would, indeed, be, to a certain extent, a natural 

 classification, from one point of view. Or we might 

 divide mankind more arbitrarily into species according 

 to height, weight, disposition, or attainments, which 

 would be a classification into artificial species. A natural 

 system differs from the foregoing in showing real rela- 

 tion through common ancestry. To be sure, in the 

 classification of the human race according to occupations, 

 a true relation through common ancestry is shown, for 

 the progenitors of the human race undoubtedly all had 

 much the same occupation, from which all subsequent 

 pursuits have been evolved; but a natural system of 

 division must do more than indicate a real connection 

 by evolutionary succession it must show that partic- 

 ular connection or group of connections which is most 

 vital to the nature or being of the thing classified. In 

 fact that which is used as a basis for a natural classifica- 

 tion must be that which is most fundamental to the 

 being classified, and e'ach stage in division must be based 



