520 
R. RUGGLES GATES 
parent and offspring, we may state that heredity consists in the 
perpetuation of the differences between related organisms. Inheritance 
is then the process by which these differences are perpetuated from 
generation to generation. This manner of statement is particularly 
useful when we contrast heredity and variation from the evolutionary 
point of view. For we may then define variation as the process by 
which new differences arise, and inheritance as the process by which 
they are perpetuated. 
These definitions have much more biological usefulness than may 
at first appear. They enable us to compare the phenomena of heredity 
and variation from a different point of view. 
Though the above definition of variation applies to all kinds of 
variations, yet it has in view particularly mutations, which are com- 
pletely inherited. We may classify variations as regards their herit- 
ability, into three classes: (i) those which are completely inherited, 
(2) those which are non-inherited, (3) those which are partially 
inherited. These three classes of variations must then have very 
different evolutionary significance. It is obvious that completely 
inherited variations, or mutations, are immediately effective for 
evolution or at least for species-formation, though it does not neces- 
sarily follow that they are of greatest evolutionary significance. It is 
not necessary to enter into this question here, nor discuss the relative 
importance of classes (i) and (3),^ — matters which are still to a large 
extent in dispute. We may, however, point out that bathmic varia- 
tions, such as the rectigradations of Osborn, may come in a different 
category still as regards their relation to evolutionary processes. 
The relative evolutionary significance of mutations and continuous 
or partially-inherited variations can be determined in part by com- 
parison of specific differences in particular groups with the mutations 
which occur in those groups. Although a mere beginning has as yet 
been made in this direction, yet it would appear that mutations have 
probably played a Jarger part in specific differentiation in some groups 
than in others. But it will be a long time before these questions can be 
definitely decided. We may, however, affirm without doubt that in 
many groups mutations have played an important and even a pre- 
ponderant part in species formation. Therefore whatever place may 
ultimately be assigned to mutation in the hierarchy of evolutionary 
factors in relation to the paleontological history of organisms, it has 
undoubtedly played an important role in the process of speciation. 
