1480 CLASSIFICATION AND SUCCESSION OF PLANTS. 



Dawson observes, " the oldest plants that we as yet certainly know 

 are Algae, and with these there are plants apparently with the struc- 

 ture of Thallophytes, but the habits of trees, which for want of a 

 better name I may call Protogens " [Nematodendreae]. " Plants 

 akin to the Rhizocarps also appear very early. Next in order we 

 find forests in which gigantic Ferns, Lycopods, and Horsetails pre- 

 dominate, in association with Pines. Succeeding these we have a 

 reign of Gymnosperms ; and in later formations we find the higher 

 Phanerogams dominant. Thus there is an advance in elevation and 

 complexity along with the advance in geological time, but connected 

 with the remarkable fact that in earlier periods low groups attain to 

 an elevation unexampled in later epochs, when their places are 

 occupied with plants of higher types." 1 



The age of Algae and Nematophytes corresponds with the earlier 

 portion of the Palaeozoic period, while Pteridophytes, with some 

 Gymnosperms, are the dominant forms of the later Palaeozoic de- 

 posits. The Mesozoic period may be termed the " Age of Gymno- 

 sperms," while the Angiosperms assume the leading place in the 

 Tertiary. The floras of the great geological periods, however, shade 

 gradually into one another, and no hard-and-fast lines can be drawn 

 between them. Thus, we have already seen that Gymnosperms 

 make their first appearance in the Upper Palaeozoic rocks, these 

 ancient types being the precursors of the characteristically Gymno- 

 spermous flora of the Mesozoic period. In a similar manner the 

 Upper Cretaceous flora, by its great development of Angiosperms, 

 is more nearly related to the Tertiary than to the preceding Jurassic 

 flora. Moreover, the evolution of the flora of different regions of 

 the earth by no means advanced pari passu with the evolution of 

 the fauna — a striking example of this fact being afforded by the 

 Lower Gondwana beds of India and the equivalent deposits of 

 Australia, in which, reckoning from a European standpoint, we find 

 a full-blown Mesozoic flora coexisting with a Palaeozoic fauna. 



Taking a brief historical retrospect of the distribution of plants in 

 time, we have no direct evidence of the existence of vegetable life 

 during the period represented by the Archaean rocks. There is, 

 however, a strong probability in favour of Sir William Dawson's 

 view that the extensive accumulations of graphite associated with 

 the Laurentian limestones of Canada are really of the nature of 

 metamorphosed vegetable matter, or that they have been derived 

 in the first instance from plants. In deposits of Cambrian age 

 most of the supposed remains of plants belong to the obscure and 

 difficult group of fossils commonly spoken of as u Fucoids," and 

 supposed to be referable to the Sea-weeds. So far as the " Fucoids " 



1 Some slight verbal alterations have been made in this quotation. 



