THE EVOLUTIONARY CAREER 319 



physical conditions under which organisms may exist. Yet 

 we know that hfe is materiahzed in a very few kinds of matter 

 and we cannot anticipate any material evolution in even those 

 thousands of millions of years that we contemplate. We know, 

 with some confidence that geological changes, cyclical, cosmic 

 and seasonal changes, gravity, solar radiation, temperature limits, 

 etc., will be very much the same for thousands of millions of 

 years to come as they were in the past history of our planet. And 

 that being so we cannot anticipate any radical changes in the 

 life that exists on the earth. 



For even the 600 millions of years that have elapsed since the 

 Cambrian deployment are a significant fraction of the total 

 evolutionary period that we envisage. In that time about a dozen 

 great types of living things have evolved — and having once evolved 

 they have persisted. No new phylum has come into existence 

 and no phylum has suff"ered extinction. The vicissitudes of 

 evolution have been expressed only as the episodes and these 

 we may regard as non-essential structural and functional develop- 

 ments that represent what w^e have called excess-value in trans- 

 formism. They have not been, like the great phyla, persistent 

 and successful processes. They are simply what we have called 

 them, episodes in a main theme that continues. It may not be 

 at all foolish to maintain that already the full possibilities of the 

 materialization of life on the earth have been realized. 



107^. Man. But it is not certain that we should regard the 

 evolution of man as only an episode. We might take that view 

 if we were to restrict our speculations to man, the mammal, 

 and refuse to consider man as himself a transformist agency. As 

 an animal we should have no reason for thinking that man is 

 not exposed to all the risks that other highly specialized animals 

 have endured in the past. We should not have any confidence, 

 for instance, in supposing that man, the Mammal, may always 

 retain dominance over the equally ubiquitous and formidable 

 insects. 



But we do not seem obliged to impose any limits (other than 

 those inherent in the passage of nature) on man's power of 

 influencing the changes that proceed on the earth : we have 

 seen, for instance, that even now man can aff"ect (infinitesimally, 

 that is) the course of evolution of the bodies in the solar system. 

 We know that during the very short period of a few thousands of 



