I02 A Factor in Evolution 



sweeping change in the environment, of such a kind that 

 only the variations lying near the extreme n can accommo- 

 date to it and live to reproduce. The next generation would 

 then show variations about the mean n. And the chances 

 of fossils from this generation, and the subsequent ones, 

 would be of creatures approximating n. Here would be a 

 great discontinuity in the chain of descent and also a wide- 

 spread prevalence of variations seeming to be in a single 

 direction. This seems especially likely when we consider 

 that the paleontologist does not deal with successive gen- 

 erations, but with widely remote periods, and the smallest 

 lapse of time which he can take cognizance of is long 

 enough to give the new mean of variation, n, a lot of gen- 

 erations in which to multiply and deposit its representative 

 fossils. Of course this would be only the action of natural 

 selection upon * preformed ' variations in those cases which 

 did not involve positive changes, in structure and function, 

 acquired in ontogenesis; but in so far as such ontogenetic 

 accommodations were actually at hand, the extent of 

 difference of the ;/-mean from the ,r-mean would be 

 greater, and hence the resources of explanation, both of 

 the sudden prevalence of the new type and of its dis- 

 continuity from the earlier, would be much increased. 

 This additional resource is due to the organic selection 

 factor. 1 



We seem to be able also to utilize all the evidence 

 usually cited for the functional origin of specific characters 

 and groupings of characters. So far as the Lamarckians 

 have a strong case here, it remains as strong if organic 

 selection be substituted for the 'inheritance of acquired 



1 A synopsis of the applications of this principle is given below, in Chap, 

 XIII. 



