102 ZOOLOGY. 



vironment. There is thus a need of terms to express the de- 

 gree of difference which, through these influences, finally char- 

 acterizes the descendants even of common ancestors. Such 

 groups of forms are usually known as varieties or subspecies 

 of the original type from which they all sprang. Thus in the 

 human race while all men are considered as belonging to one 

 common type and possibly derived from the same human 

 ancestors there is enough difference between the American 

 Indian and the Caucasian to make it necessary to distinguish 

 them as different varieties. Many of our widely distributed 

 animals as the dog, the horse, the common fox have varieties 

 which are readily distinguishable. When the causes which 

 produce varieties have been at work long enough to eliminate 

 the intermediate forms which are often found connecting the 

 varieties, and to secure a close adaptation of the varieties to 

 the environment, the term species is applied to what were for- 

 merly called varieties. Species thus merely represent the fur- 

 ther progress of individual differentiation and adaptation to 

 the different modes of life which give rise to variation in 

 individuals that is, to varieties. A species of animals may 

 again split up by the action of the forces mentioned (and other 

 conditions which have not been mentioned) into new varieties 

 and finally into new species. It is believed that the present 

 diversity of animal and plant life has come about from a much 

 more limited number of kinds of ancestors by a method essen- 

 tially such as that described above. Varieties of the same 

 species usually cross freely and the offspring are known as 

 mongrels. The individuals of different species as a rule cross 

 less freely and when they do cross their offspring are called 

 hybrids. Hybrids are often sexually infertile. 



The genus is related to species somewhat as the species to 

 the varieties which compose it. A genus embraces those kin- 

 dred species which show a high degree of relationship among 

 themselves. The characters which serve to distinguish differ- 

 ent genera are more fundamental than those by which we 

 recognize varieties or species, and argue a more extended time 

 in the differentiation of genera than is required for species 



