THE SERIES OF LIVING SPECIES. 3 II 



the ancestors of man, and of the higher mammals, appear 

 one after the other in the earth's history ; first fishes, then 

 amphibians, later the lower, and at last the higher mam- 

 mals. Here, therefore, the embryonic development of 

 the individual is completely parallel to the palseontological 

 development of the whole tribe to which it belongs, and this 

 exceedingly interesting and important phenomenon can be 

 explained only by the interaction of the laws of Inheritance 

 and Adaptation. 



The example last mentioned, of the parallelism of the 

 palseontological and of the individual developmental series, 

 now directs our attention to a third developmental series, 

 which stands in the closest relations to these two, and which 

 likewise runs, on the whole, parallel to them. I mean that 

 series of development of forms which constitutes the object 

 of investigation in comiioarative anoiomy, and which I will 

 briefly call the systematic developTnental series of species. 

 By this we understand the chain of the different, but re- 

 lated and connected forms, which exist side hy side at any 

 one period of the earth's history; as for example, at the 

 present moment. While comparative anatomy compares the 

 different forms of fully-developed organisms with one 

 another, it endeavours to discover the common prototypes 

 which underlie, as it were, the manifold forms of kindred 

 genera, classes, etc., and which are more or less concealed by 

 their particular differentiation. It endeavours to make out 

 the series of progressive steps which are indicated in the 

 different degrees of perfection of the divergent branches of 

 the tribe. To make use again of the same particular in- 

 stance, comparative anatomy shows us how the individual 

 organs and systems of organs in the tribe of vertebrate 



