THE BIOLOGICAL RELATIONS OF FOSSILS. 6 1 



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 palae- 

 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. 



