THE SERIES OF LIVING SPECIES. 311 
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 paleontological 
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 
paleontological 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 comparative anatomy, and which I will 
briefly call the systematic developmental series of species. 
By this we understand the chain of the different, but re- 
lated and connected forms, which exist side by 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 
