ASPECTS OF MODERN BIOLOGY 547 



insects and plants of the Florissant Miocene, the more convinced I 

 become that, speaking broadly, the extinct genera and higher groups 

 are not the ancestors of any now living, but represent types which have 

 failed, like the mammoth ; while the real representatives of the modern 

 biota show that there has been singularly little forward evolution in 

 the course of perhaps a million years. Many of these are totally extinct 

 in Colorado, but live elsewhere; thus the redwood differs little from 

 that of California, while the wonderfully delicate and fragile Halter, 

 belonging to a family no longer living in North America, is closely 

 related to a living species of Persia. 



Hence the experimental researches of De Vries and others, proving 

 that mutation is a relatively common phenomenon among plants, 

 prove perhaps too much. If change is so easy, why so little change, 

 and that in the face of a radical change in temperature and moisture? 

 It seems, indeed, that " elementary species " have always been produced 

 in greater or less abundance, but by a sort of oscillation less related to 

 the forward march of evolutionary activity than we might at first sup- 

 pose. The ability to produce heritable segregates, especially in the 

 face of adverse or strange conditions, is clearly of advantage, as giving 

 new chances for spread or survival. Thus in the long run the tendency 

 to break into " elementary species " would in many cases be favored 

 by natural selection, without any necessity for each one of these, or 

 even the majority, being directly related to a particular environment. 

 There is no reason, apparently, why this should not continue for ages 

 as an oscillation-process, a segregation in space rather than in time, 

 producing thousands of species without overstepping the limits of the 

 general group, or perhaps advancing at all in complexity. The mol- 

 luscan genus Ostrea, the oysters, may be taken as an example of this; 

 indeed, the modern oysters scarcely do justice to their Cretaceous ances- 

 tors. When it was generally held that species were created by divine 

 fiat, it naturally appeared that he who should explain the origin of 

 species might be given the rest without further charge. We are coming 

 to see that there are diverse problems involved, and while the whole 

 matter may well be locked up in the evolution of any single species, or 

 indeed of any single cell, we begin to doubt whether we really possess 

 the key. 



Speaking philosophically, progressive or orthogenetic evolution — 

 the existence of which no naturalist has any ground for doubting — must 

 have a cause external to itself. All probability favors the idea that 

 this did not operate once for all, but has continued in action throughout 

 the ages. It may be found, perhaps, in the susceptibility of the hered- 

 itary mechanism to environmental influences of particular kinds, the 

 nature of which remains for the present obscure. These reactions 

 would fall under the operation of natural selection from the very 



