PROGRESS OE LIEE. 599 



In all cases, however, there was an unfolding of a type, — an exhi- 

 bition of it through the successive appearance of new groups, in which 

 groups characteristics before only foreshadowed, or existing only in 

 potentiality, come out into full expression. The early general or com- 

 prehensive type thus becomes in a sense specialized, or represented 

 in numbers of special groups. In the case of Fishes, the type, when 

 the Teliosts appeared, came forth in its purity, deprived of the 

 Reptilian features of the Ganoids (marked in their vertebrae, teeth, 

 air-bladder, and other parts) and developed in those points which 

 make up the true Fish. Moreover, the Fish-type was at the same 

 time represented under a diversity of tribes, and an extraordinary 

 variety of shapes, -normal and abnormal, high and low (some almost 

 of the low grade of a Polyp), which was in great contrast with the 

 uniformity of structure and limited variation of form in the Ga- 

 noids. Nature thus revels in exuberance when displaying a type 

 after its true level is attained. 



In this kind of progress there is naturally expansion towards 

 inferior as well as superior grades : it is not out of harmony with 

 the system that Echinoderms should have existed before Polyps, 

 Tree-ferns and Lepidodendrids before Mosses, Lacertians and 

 Crocodiles before Snakes, or Herbivores and Carnivores, before 

 Sloths. 



When a type had passed its culmination, there was sometimes a 

 very marked decline in the character of the species that preceded its 

 final extinction. Examples of this have been referred to in the 

 last of the Leptamce that occurred in the Mesozoic (p. 450). and the 

 multiplication of uncoiled forms of the Ammonite family which 

 took place in the Cretaceous (p. 472). 



This law of specialization — the general before the special — is the 

 law of all development. The egg is at first a simple unit ; and, 

 gradually, part after part of the new structure is evolved, that which 

 is most fundamental appearing earliest, until the being is complete 

 in all its outer and minor details. The principle is exhibited in the 

 physical history of the globe, — which was first a featureless globe of 

 fire, then had its oceans and dry land, in course of time received 

 mountains and rivers, and finally all those diversities of surface 

 which now characterize it. Again, the climates began with uni- 

 versal tropics ; gradually, zones were apparent ; and at last the 

 diversity of the present day. 



But there is a wide distinction in the kind of specialization which 

 starts from a simple unit like an egg, and that proceeding from com- 

 prehensive types among plants and animals. The one is diversity 

 out of memberless simplicity ; the other, diversity from a unit of 



