METHOD IN THE HISTORY OF LIFE. 317 



of the invertebrate sub-kingdoms ; but among the orders 

 of the several classes and the classes of the Vertebrates we 

 find generally a progress from lower to higher in the order 

 of introduction. 



But there is another principle, complementary to this, 

 which needs to be united to it in order to present us with 

 a true view of Nature's method. There has generally been 

 a downward as well as an upward unfolding of each type 

 from the central forms in which it was first embodied. 

 Trilobites, the first representatives of the Crustacean type, 

 belong indeed to the lowest group, but do not lie at the 

 bottom of the group — the lower members, as well as the 

 higher groups, coming into being at subsequent periods. 

 The earliest reptiles were not the lowest of the Amphibi- 

 ans, but Labyrinthodonts, the highest Amphibians; and 

 from this starting-point the reptilian type expanded both 

 upward and downward. Vertebrates began, not with the 

 lowest fishes, but with a grade of fishes above the mean 

 level of the type in the possession of several reptilian char- 

 acteristics. From here the type rose still higher to the 

 strongly sauroid forms, and descended to the Teliosts, or 

 typical fishes, with their aberrant and degraded forms — the 

 lamprey and the lancelet. We shall arrive, therefore, at 

 the truest expression of the plan of Nature in reference to 

 the succession of organic beings by saying that each type 

 was first introduced at a nodal point, from which the 

 stream of development proceeded in both directions — the 

 lowest forms in many instances being reached only in the 

 modern age ; so that, in some cases, after the culmination 

 of a type, it has suffered a degeneration into the lower 

 grades already passed. 



Another fact strikes us in a review of the succession of 

 life in past time. Life has presented itself not so much in 

 a series of sharply-restricted organic forms, rising or de- 



