396 



PALAEOZOIC TIME. 



They are called by Agassiz, who early recognized their nature, syn- 

 thetic types, — a term not here used, as it implies that they corre- 

 spond to a combination of what was before separate, rather than to 

 one yet undivided. Guyot calls them undivided types. 



(3.) The starting-point of a class or other division of the kingdoms 

 of life was often from the top of its lower division and the bottom of 

 the next above, or from an intermediate level between the two. 



Dividing plants into Cryptogams and Phaenogams ; Crustaceans 

 into Entomostracans, the lower, and Malacostracans, the higher 

 group, as done by Cuvier; Eeptiles into Amphibians and true 

 Eeptiles ; Vertebrates into water-breathing and air-breathing : we 

 observe that — 



The earliest land-plants were the highest of the Cryptogams and 

 the lower of the Phsenogams (the Gymnosperms), together with 

 groups that are intermediate and of the nature of the comprehen- 

 sive types above described (see p. 333). The vegetable kingdom 

 began with the lowest of its tribes, the Algse, or Sea-weeds, and pro- 

 bably with the lowest of sea-weeds as far back as theAzoic age. But 

 when terrestrial plants were to appear, there was not a series rising 

 in grade from these sea-weeds up to the higher species of the land. 

 On the contrary, the lower Cryptogams of the tribes of Mosses, 

 Hepaticse, and even the fresh-water Confervee, were passed by, and 

 the new species were those at the top of the order of Cryptogams, 

 with others superior to the modern types of this order. 



The early Crustaceans, Trilobites, belonged either to the top 

 of the Entomostracans or to the bottom of the higher group, and 

 presented features of both. (See p. 202.) 



The earliest Eeptiles were not the lowest of Amphibians ; but 

 along with some of the inferior species of this division there were 

 Labyrinthodonts, a family above the true level of the Amphibians, 

 together with Lacertians, and also swimming Saurians, species belong- 

 ing to the inferior divisions of True Eeptiles. 



Vertebrates commenced, not with the lowest fishes, but with a 

 group above the true level of the fish, in a type which included 

 several characteristics of the higher class of Eeptiles (p. 302). 



The evidence in the rocks thus sustains the conclusion that many 

 of the groups began, not with their lowest species, — that is, those at 

 the bottom of the lower group, — but, on the contrary, with those at 

 the top of the lower group and near the bottom of a higher, or from 

 some intermediate point between the two. In this way a remarkable 

 harmony was given to the fauna and flora of an age-. The flora of 

 the Carboniferous is a fine example of this principle. 



This harmony was sometimes further promoted by adding to one 



