118 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 
ef 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 
