20 LHE POPULAR SOIENCE MONTHLY 
seem nearly always destined to die out, being supplanted by the de- 
scendants of simpler and more plastic forms. Supposing the career of 
man to resemble that of other specialized mammalia, he might be ex- 
pected to have before him perhaps another hundred thousand years, 
and then in all probability the end of the world, so far as he was con- 
cerned with it as an animal. Even on this hypothesis, he would have 
as much occasion to prepare for his terrestrial future as a young child 
has for its adult life, but there are very good reasons for supposing 
that the fate of man need not necessarily be the same as that of the 
animals to which he is most nearly allied. Prior to the existence of 
man, living beings might have been divided roughly into two groups, 
those related to very simple or unchanging environments, such as the 
amoeba or the oyster, and those specially adapted to complex conditions, 
such as the yucca moth and the giraffe. The former have proved suc- 
cessful through their very simplicity, have been saved by their lack of 
progress; the latter are nature’s masterpieces, often destined, as such 
things are, to go out of fashion. Any single man may be taken as a 
rather extreme example of the latter type; he is extraordinarily de- 
pendent upon a special set of conditions, but the race as a whole is rela- 
tively independent, and without sacrificing anything of its organic com- 
plexity, is able to meet and overcome the dangers which have destroyed 
so many of the higher mammals. If with this man can secure a genuine 
but moderate progress in his fundamental organism, not sufficient to 
break the continuity of tradition or destroy his essential specific unity, 
he may be assured a career such as no mammal ever had before. 
The causes of the extinction of other animals have been principally 
related to climate, food and natural enemies, including here the germs 
of disease. With regard to climate, man at first, through racial differ- 
entiation, became adapted to everything from tropical heat to arctic 
cold; but here he was on the way to split up into a number of distinct 
species. Now through devices of housing and clothing he can almost 
create climatic environments for himself, and so single races, or mix- 
tures of races, are to be found nearly everywhere. At the same time, 
like the bird, he knows how to migrate when necessary, so that he will 
never be destroyed by changes confined to a single continent or even 
hemisphere. 
In the case of food, he is relatively unspecialized, and no doubt his 
omnivorousness has greatly aided his spread over the globe. So long 
as he had to depend upon the supplies furnished gratis by nature this 
was a necessary condition of his cosmopolitanism ; but now that he can 
so largely control his food supply, and can carry any given product to 
the opposite end of the earth, it is a question whether there will not be 
a distinct gain in a return to primitive simplicity in diet. 
Of natural enemies, the grosser and more tangible kind, like the 
