Natural History of the United States. 205 



while prophetic types exemplify structural combinations observed 

 at a later period, in two or several distinct types, and are, more- 

 over, 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 pedunculated Crinoids. 



Another combination is also frequently observed among ani- 

 mals, when a series exhibits such a succession as exemplifies a 

 natural gradation, without immediate or necessary reference to 

 either embryonic development or succession in time, as the 

 Chambered Cephalopods. Such types I call progressive types. 



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 their structural complications lean towards other 

 combinations fully realized in a later period, while 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 Ichtliyosauri are more distinctly 

 synthetic than prophetic types, while Pterodactyles have more 

 the character of prophetic types ; so are also Echinocrinus with 

 reference to Echini, Pentremites with reference to 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, in the course 

 of this work, to present ample details upon this subject. 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." 



Another example may be taken from a section giving the views 

 of Agassiz, on the much debated question of the date of succes- 

 sion of fossil animals in its relations to their grade in nature. 



PARALLELISM BETWEEN THE GEOLOGICAL SUCCESSION OE ANIMALS 

 AND PLANTS AND THEIR PRESENT RELATIVE STANDING. 



" The total absence of the highest representatives of the animal 

 kingdom in the oldest deposits forming part of the crust of our 



