38 BIOLOGICAL SOCIETY <>F WASHINGTON. 



differs in being exceedingly irregular in the intervals of branch- 

 ing. Expressing the process in time we observe that vast 

 periods pass in waiting for the working out of the most simple 

 principle, which, when once hit upon, produces a complete 

 and rapid revolution in an entire department of life. I can 

 liken it in this respect only to the progress of mankind as 

 brought about by great mechanical inventions made at irregu- 

 tar intervals and producing undreamed-of revolutions in the 

 whole industrial frame- work of society. The length of the 

 stationary periods in biologic evolution is determined by no 

 fixed law. When a type of structure has advanced as far as it 

 is capable of developing it remains stationary as long as noth- 

 ing interferes with its continuance. If no change should 

 take place in its environment it might continue for an 

 indefinite period. As, by hypothesis, it can advance no 

 farther it can only vary in the direction of deterioration or 

 extinction. The type of structure once fixed can never 

 change. Only the degree of vigor, luxuriance, or abund- 

 ance can undergo modification. Deterioration is everywhere 

 illustrated by the present cryptogamic vegetation. The 

 Carboniferous forests of Eepidodendron and Calamites are 

 represented by our little club-mosses and scouring rushes, al- 

 though they must have descended from trunk lines which had 

 not yet acquired the exogenous structure. Extinction is ex- 

 emplified by the absence of exogenous cryptogams in the liv- 

 ing flora, as also of most of the later cycadean and coniferous 

 types. There are several interesting cases of partial and rap- 

 idly approaching extinction. Among such may be mentioned 

 the maidenhair-tree, the mammoth and redwood trees, and 

 also, it would seem, the tulip and plane trees, all of which in 

 their turn dominated the vegetable kingdom, but now, though 

 undiminished in vigor or structural perfection, have been re- 



