Il8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



classifications were thus largely artificial and served principally as 

 convenient methods of arrangement, description and cataloging. 

 Since the time of the development of the theory of descent with 

 modifications by Lamarck (1809) and Darwin (1859), there has been 

 an attempt to base the classification on relationships. Very nearly 

 related animals are put into the same species. They are related 

 because they descend from a common ancestry, and that common 

 ancestry could not in most cases have been very ancient, otherwise 

 evolution within the group would have occurred and the species would 

 have been split into two or more species. Species that are much 

 alike are included in one genus, being thus marked off from the species 

 of another genus. The similarity of the species of a genus is held to 

 indicate kinship, but since there is greater diversity among the indi- 

 viduals of a genus than among the members of a species, the common 

 stock from which the species of a genus have sprung must have existed 

 at an earlier time, in order that evolution could bring about the degree 

 of divergence now observed. In like manner, a family is made up of 

 genera, and their likeness is again a sign of affinity. But to account 

 for the greater difference between the extreme individuals belonging to 

 a family, evolution must have had more time, that is, the common 

 source of the members of a family must have antedated the common 

 source of the individuals of a genus. Orders, classes, and phyla are 

 similarly regarded as having sprung from successively more remote 

 ancestors, the time differences being necessary to allow for the differ- 

 ences in the amount of evolution. This statement is in general correct. 

 However, since evolution has probably not proceeded at the same rate 

 at all periods, nor in all branches of the animal kingdom at any one 

 time, the time relations of the groups of high or low rank must not be 

 too rigidly assigned. Thus certain genera, in which evolution has been 

 slow, are probably much older than some families in which evolution 

 has been rapid. It is not improbable, also, that some genera are quite 

 as old as the families which include them; but in no case can they be 

 older. Furthermore, different groups are classified by taxonomists of 

 different temperaments, so that groups of a given nominal rank may 

 be much more inclusive (and hence older) in one branch of the animal 

 kingdom than in another. On the whole, nevertheless, the groups of 

 higher rank have sprung from ancestry more remote than that of the 

 groups of lower rank. 



The means of recognizing the kinship implied in classification 

 permit some differences of opinion. It is recognized that likeness in 



