240 WE AT IS LIFE 



much general similarity and much diversity in detail. 

 Always, of course, earlier, lower life-forms persisted 

 as conditions permitted, "adapting" themselves to 

 the limit of possibility. The possibilities of adapta- 

 tion, however, necessarily were limited. "Adapta- 

 tion" as a factor in evolution has been greatly 

 exaggerated. That the life-forms which escaped 

 destruction when the wholesale loss of life, with 

 extinction of many species, occurred, had any part 

 in the evolution of higher forms that followed these 

 destructive periods, seems most unlikely. Most, if 

 not all such surviving forms in all probability had 

 become distinct forms long before the disturbances 

 set in. And, according to my theory, a ''species'' 

 (such as the life-forms, mentioned by Osborn, that 

 remained static for millions of years — see pp. 207,208) 

 must be conceived to be the end of a series; not a step, or 

 link, to higher forms. 



A dual-system such as the theory conceives every 

 life-form to be, is a strictly limited system, a system 

 limited by its own constitution. Any occasion, every 

 occasion, that gave rise to life, gave rise to life-forms 

 that were strictly limited as to possibilities of de- 

 velopment, and of variability. Inevitably, when the 

 limit of these possibilities was reached, sooner or 

 later in geologic time, the "pattern" became rigid, 

 no further development, or evolution, being possible. 



