116 ESSAY ON CLASSIFICATION 



ture peculiarities which at later periods are only observed separately 

 in different, distinct types. Sauroid Fishes before Reptiles, Pterodac- 

 tyles before Birds, Ichthyosauri before Dolphins, etc. 



There are entire families among the representatives of older peri- 

 ods of nearly every class of animals, which in the state of their perfect 

 development exemplify such prophetic relations, and afford within 

 the limits of the animal kingdom, at least, the most unexpected evi- 

 dence that the plan of the whole creation had been maturely consid- 

 ered long before it was executed. Such types I have for some time 

 past been in the habit of calling prophetic types. The Sauroid Fishes 

 of the past geological ages are an example of this kind. These Fishes, 

 which have preceded the appearance of Reptiles, present a combina- 

 tion of ichthyic and reptilian characters not to be found in the true 

 members of this class, which form its bulk at present. The Pterodac- 

 tyles which have preceded the class of Birds, and the Ichthyosauri 

 which have preceded the appearance of the Crustacea are other ex- 

 amples of such prophetic types. These cases suffice for the present to 

 show that there is a real difference between embryonic types and 

 prophetic types. Embryonic types are in a measure also prophetic 

 types, but they exemplify only the peculiarities of development of 

 the higher representatives of their own types; while prophetic types 

 exemplify structural combinations observed at a later period in two 

 or several distinct types and are, moreover, not necessarily embryonic 

 in their character, as for example, the Monkeys in comparison to 

 Man; while they may be so, as in the case of the Pinnate, Plantigrade, 

 and Digitigrade Carnivora, or still more so in the case of the pedun- 

 culated Crinoids. 



Another combination is also frequently observed among animals 

 when a series exhibits such a succession as exemplifies a natural gra- 

 dation, without immediate or necessary reference to either embryonic 

 development or succession in time, as the Chambered Cephalopods, 

 Such types I call progressive typesM'^ 



Again: a distinction ought to be made between prophetic types 

 proper and what I would call synthetic types, though both are more 

 or less blended in nature. Prophetic types proper are those which in 



^"- Agassiz, "Progressive, Embryonic, and Prophetic Types," Proceedings, AAAS, II 

 (1850). 432-438. 



