286 GENETICS IN RELATION TO AGRICULTURE 



their germ cells are similar. Hence they breed true in the main but 

 occasionally throw the new combinations of diverse elements which have 

 come to be known as "mutants." 



In conclusion, it may be well to state our reasons for restricting the 

 term, mutation, to those changes in specific factors, which result in the 

 appearance of new Mendelizing characters. This term was used by de 

 Vries to designate a more or less comprehensive change which appeared 

 suddenly, without warning, giving the impression that a full-fledged new 

 species had sprung from a pure, constant, old species much as Athena 

 sprang from the head of Zeus. We cannot conceive of new species 

 originating in this way except in certain exceedingly rare cases, which 

 fall under the two categories already described and illustrated, viz., 

 (1) single factor mutations having such a profound manifold effect that 

 the new form would be generally recognized as a distinct species, and (2) 

 chromosome aberrations during mitosis or meiosis. We have found that 

 the majority of the new forms derived from (Enothera lamarckiana do 

 not fall into either of these categories and that the most reasonable 

 explanation of their origin is based on the assumption that (E. lamarck- 

 iana is of hybrid origin. Therefore, if the term, mutation, is to retain 

 the meaning originally given it by de Vries, we cannot continue to classify 

 the majority of new (Enotheras or other organisms resulting from 

 hybridization as mutations. 



On the other hand, the fact that most discontinuous, inheritable varia- 

 tions are caused by alterations in genetic factors and that these factor 

 mutations play an important role as one means for organic evolution, 

 seems to justify their recognition as mutations in the strict sense. By 

 limiting the meaning of mutation as we propose all the objectionable im- 

 plications previously connoted by the term are removed. The desir- 

 ability of accomplishing this has been indicated by Agar, who states: 



" The greatest opposition to modern views of genetics has come from those 

 who consider that they have taken away the philosophical basis of the theory 

 of evolution and especially of the evolution of adaptation. For, while mutation 

 could quickly bring about specific diversity, the evolution of complex adaptive 

 structures is undoubtedly most easily grasped when the inheritable variations 

 presented to natural selection are minute and abundant. This difficulty, though 

 real, would undoubtedly have assumed smaller proportions had it not been for 

 the natural fact that the earliest mutations studied were large morphological 

 ones, and consequently that these have become fixed in many minds as types of 

 mutational change." 



There is now abundant evidence that genetic diversity is expressed 

 in minute morphological and physiological differences, and hence that 

 mutations produce those small inheritable differences logically required 

 for the explanation of adaptation through natural selection. 



