The Theory of Evolution 31 



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 transmutation 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. 



