178 ESSAY ON CLASSIFICATION. 



complications lean towards other combinations fully rea- 

 lized in a later period, wliile synthetic types are those 

 which combine in a well balanced measure features of 

 several types occurring as distinct, only at a later time. 

 Sauroid Fishes and Ichthyosauri are more distinctly syn- 

 thetic than prophetic types, while Pterodactyles have more 

 the character of prophetic types; so also are the genera Echi- 

 nocrinus with reference to the Echini, Pentremites with re- 

 ference to the Asterioids, and Pentacrinus with reference to 

 Comatula. Full illustrations of these different cases will 

 yet be needed to render obvious the importance of such 

 comparisons ; and I shall not fail to present ample details 

 upon this subject in my Contributions to the Natural His- 

 tory of the United States, now in course of publication. 

 Enough, however, has already been said to show that the 

 character of these relations among animals of past ages, 

 compared with those of later periods or of the present 

 day, exhibits more strikingly than any other feature of 

 the animal kingdom, the thoughtful connection which 

 unites all living beings, through all ages, into one great 

 system, intimately linked together from beginning to end. 



SECTION XXVII. 



PARALLELISM BETWEEN THE STRUCTURAL GRADATION OF ANIMALS 

 AND THEIR EMBRYONIC GROWTH. 



So striking is the resemblance of the young of higher 

 animals to the full-grown individuals of lower types, that 

 it has been assumed by many writers that all the higher 

 animals pass, during the earlier stages of their growth, 

 through phases corresponding to the permanent constitu- 

 tion of the lower classes. These suppositions, the results 



