62 PRINCIPLES OF PALEONTOLOGY. 



gression and development in the types of animal life which 

 characterize the geological ages. If we take the earliest-known 

 and the oldest examples of any given group of animals, it can 

 sometimes be shown that these primitive forms, though in 

 themselves highly organized, possessed certain characters such 

 as are now only seen in the young of their existing representa- 

 tives. In technical language, the early forms of life in some 

 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 embryonic char- 

 acters and this " comprehensiveness " of structural type are signs 

 of what a zoologist considers to be a comparatively low grade of 

 organization ; and the prevalence of these features in the earlier 

 forms of animals is a very striking phenomenon, though they 

 are none the less perfectly organized so far as their own type 

 is concerned. As we pass upwards in the geological scale, we 

 find that these features gradually disappear, higher and ever 

 higher forms are introduced, and " specialization " of type takes 

 the place of the former comprehensiveness. 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 

 generalization of palaeontology, 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. 



