Inheritance of Variations 77 



classed as different species, although the question of what 

 constitutes a species must be left to the judgment or fancy 

 of the individual. 



Since the work on Difflugia was done, the same methods of 

 work have been applied in our laboratory to two of its rela- 

 tives, Centropyxis aculeata and Arcella dentata. In the 

 former, Root (1918) found considerable evidence that cer- 

 tain variations within the single stock were inherited, al- 

 though the work was not carried so far as in Difflugia. In 

 Arcella, Hegner (1918) found that heritably diverse stocks 

 could be isolated by selection from a single stock multiplying 

 by fission. 



All together then, our "second degree" study of the mat- 

 ter has been rewarded by the discovery that for these animals 

 at least the situation that I sketched in my last lecture is 

 not final. In these animals we do find the diverse races present 

 under natural conditions, just as in other organisms (see 

 Figures 20 and 21); and by mere selection among these 

 diverse races we can get all sorts of apparent changes 

 which are not real changes; which are not evolution. But 

 when we take a single race and devote all our attention to 

 that alone for years, then we find that real changes do 

 occur; that the race differentiates into many races in the 

 way I have described ; that evolution visibly does occur. 



Now I told you that the other theory was the prevailing 

 one; so much is this the case that some of my readers 

 will not accept unreservedly these cases as actual changes 

 in hereditary constitution; as actual steps in evolution; on 

 the contrary they are trying to devise various possible 

 schemes by which it could be made to seem that even here 

 in Difflugia we are getting nothing but new combinations of 

 what was before present. Many such schemes have been 

 devised for explaining apparent effects of selection in higher 



