The Theory of Evolution Si 
law is not concerned with the likeness or unlikeness of the dif- 
ferent individuals that descend from each other. The theory 
of evolution includes the same idea, but in addition it has 
come to mean nowadays, that there have been changes, as 
the succeeding generations have arisen. The transmutation 
theory, and even the descent theory, have come to mean nearly 
the same thing as the theory of evolution. It is unfortunate 
that one of these terms cannot be used to signify simply the 
repetition, generation after generation, of groups of similar 
individuals. The theory of descent might be used to convey 
only this idea, but unfortunately it too has come to include 
also the idea of change. I shall attempt nevertheless to dis- 
criminate between the descent and the transmutation theory, 
and use the term descent theory when I do not wish to con- 
vey the idea of change, and Zransmutation theory when I do 
wish to emphasize this idea. 
On the transmutation theory it is assumed that a group 
(species) may give rise to one or more groups of forms differ- 
ing from their ancestors; the original group being now re- 
placed by its new kinds of offspring, or the old and the new 
may remain in existence at the same time. This process 
repeating itself, each or some of the new groups giving rise in 
turn to one or more new species, there will be produced a larger 
group of species having certain similar characters which are 
due to their common descent. Such a group of species is 
called a genus. The resemblances of these species is 
accounted for by their common descent ; but their differences 
must be due to those factors that have caused them to depart 
from the original type. We may now proceed to consider 
the evidence on which this idea of transmutation rests. 
