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 
gigsperms (dicotyls) 
cyca 
[norsetalls > 
pteridos| erms. 
=—Erape ferns 
an 
———$—$—— 
believe that the condi- a\ 3 \ 
tions brought about by ww 
planetary decline and 
“old age” would be — Bryophytes N 
the same as those at- 
tending planetary 
birth and adolescence. 
419. Explanation of 
the diagram. — An at- 
tempt to show the Fic. 511. — Diagram showing the supposed 
course of plant evolution. 
general course of plant 
evolution up to the present time is made in the accompany- 
ing diagram. The four great divisions, Thallophytes, Bryo- 
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- 
aige® 
—_Llive 'Worts 
4 : 
a. o 
\ 
Thallophytes 
