THE BIOLOGICAL RELATIONS OF FOSSILS. 61 
instances possess ‘“ embryonic” characters, though this does 
not prevent them often attaining a size much more gigantic 
than their nearest living relatives. Moreover, the ancient 
forms of life are often what is called ‘comprehensive types” 
—that is to say, they possess characters in combination such 
as we nowadays only find separately developed in different 
groups of animals. Now, this permanent retention of embry- 
onic characters and this ‘comprehensiveness” of structural 
type are signs of what a zoologist considers to be a compara- 
tively low grade of organisation ; and the prevalence of these 
features in the earlier forms of animals is a very striking phe- 
nomenon, though they are none the less perfectly organised so 
far as their own type is concerned. As we pass upwards in 
the geological scale, we find that these features gradually dis- 
appear, higher and ever higher forms are introduced, and 
“specialisation ” of type takes the place of the former com- 
prehensiveness. We shall have occasion to notice many of 
the facts on which these views are based at a later period, and 
in connection with actual examples. In the meanwhile, it is 
sufficient to state, as a widely-accepted generalisation of pale- 
ontology, that there has been in the past a general progression 
of organic types, and that the appearance of the lower forms 
of life has in the main preceded that of the higher forms in 
point of time. 
