CRYPTOGAMS 



361 



to conceive of a state, as our planet approaches the period 

 of cosmic debility and decay, when the conditions of existence 

 may become progressively more and more unfavorable. In 

 this case the course of evolution would be reversed, the higher 

 types gradually dying out as the struggle for life became 

 more severe, and the tendency would be constantly toward 

 lower and simpler forms, until finally all life would become 

 extinct on our planet. 

 We have no right, how- 

 ever, to assume that 

 during such a course of 

 retrogressive evolution 

 the same forms would 

 be repeated in reverse 

 order as have already 

 appeared, because 

 there is no reason to 

 believe that the condi- 

 tions brought about by 

 planetary decline and 

 "old age" would be 

 the same as those at- 

 tending planetary 

 birth and adolescence. 

 419. Explanation of 

 the diagram. — An at- 

 tempt to show the 

 general course of plant 

 evolution up to the present time is made in the accompany- 

 ing diagram. The four great divisions, Thallophytes, Brj'o- 

 phytes, Pteridophytes, and Spermatophytes, are represented 

 by spaces between four horizontal lines arranged one above 

 the other in the order of their succession in time and com- 

 plexity of organization. It should be borne in mind that 

 these dividing lines are not sharply defined in nature, but 

 overlap or indent the territory between them with vary- 



Thallophytes 



Fig. 



511. — Diagram showing the supposed 

 course of plant evolution. 



