DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION IOQ 



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 con- 

 sidered as belonging to one common type and possibly derived 

 from the same human ancestors there is enough difference be- 

 tween the American Indian and the Caucasian to make it neces- 

 sary 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 formerly called varieties. Species thus merely rep- 

 resent the further progress of individual differentiation and adap- 

 tation 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. The student will realize 

 that in nature there are only individuals. There is really no 

 such thing in nature as a species. This is purely, a mental con- 

 cept of our own. 



Varieties of the same species usually cross freely. Their 

 offspring are usually fertile. The individuals of different spe- 

 cies as a rule cross less freely and when they do cross their off- 

 spring 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. 



Other terms, as families, orders, classes, phyla, are used to 



