VARIOUS THEORIES OF SPECIES-FORMING 115 



species-forming in plants has the theory of species-forming by 

 mutations, or sudden fixed changes (lesser or greater) had any 

 considerable adoption or even general attention. 



At the present moment, probably because of a strong re- 

 action against the too blind acceptance and general over- 

 emphasis of the selection doctrines, and because, too, of the 

 unusually extensive character of de Vries's experimentation and 

 observation, and his trenchant criticism of the weak places 

 in the other theories, with the generally weighty character of 

 his work and reputation, because of all this the theory of species- 

 forming by mutations has at the present moment a fairly large 

 body of adherents among reputable biologists. And yet the 

 actual evidence of tested observation on which the theory rests 

 is curiously meager. One hastens to admit, however, that 

 similar evidence for the theory of direct species-forming by 

 selection is also meager. While apparently no one has ever 

 seen a case of species-making by the natural selection of slight 

 fluctuating variations, de Vries seems to be almost the only 

 one who has observed actual cases of species-making by hetero- 

 genesis, and he has seen very few. And in the nature of things, 

 the opportunities for this kind of evidence, that is, that of actual 

 observation, ought to be much larger in the case of hetero- 

 genesis than in that of general transformation by the selection 

 of slight variations. An account of the exact character of 

 the de Vriesian mutations is included in our later chapter on 

 variation and mutation. Our readers should realize, that 

 however much they may see of this theory in present-day 

 popular scientific literature, and however strongly the case 

 may be put in favor of the mutation theory of species origin, 

 this theory is not accepted by the great body of biologists as 

 entitled to replace the Darwinian theory. 



We may close this chapter with a reference to a pregnant 

 sentence of the American paleontologist, Osborn, in a lecture 

 entitled "The Unknown Factors of Evolution": "The general 

 conclusion we reach from a survey of the whole field is that for 

 Buffon's and Lamarck's factors we have no theory of heredity, 

 while the original Darwinian factor, or Neo-Darwinism, offers 

 an inadequate explanation of evolution. If acquired varia- 

 tions are transmitted, there must be, therefore, some unknown 

 principle in heredity; if they are not transmitted, there must be 

 some unknown factor in evolution." Our present plight seems 



