276 GENETICS IN RELATION TO AGRICULTURE 



sion of the alternative forms. It seems reasonable then to explain the 

 variations in somatic expression of the genetic factors present by internal 

 changes of some sort. Frequently these variations appear as localized 

 stages in ontogeny and it is possible that internal secretions (hormones) 

 play a more important r61e in plant development than has been realized. 

 The recent experiments of Loeb on Bryophyllum calydnun indicate not 

 only the association and possible identity of root-forming and geotropic 

 substances in this plant, but also that the leaves produce growth inhibit- 

 ing substances which pass downward through the stem and which may 

 accompany or may be identical with the root-forming hormones. Cook 

 has shown that sometimes two extremely different forms of leaf occur on 

 adjacent nodes but even such abrupt transitions might result from an 

 internal reaction occurring in the interim between the development of 

 the two successive leaves. Moreover, the transition from one leaf form 

 to the other is frequently gradual as in the two series, each from a single 

 branch, shown in Fig. 113. We conclude, therefore, that most cases of 

 dimorphism in the same plant are not caused by factor mutations but 

 rather that they should be classified with those cases of " fixed dimorphism " 

 so frequently found in insects and illustrated by the earwigs in Fig. 20. 

 " Mutations" in the Evening Primroses. Credit for directing atten- 

 tion to suddenly appearing new forms of animals and plants both as 

 material for origin of species and for improvement of domesticated races 

 belongs to the Dutch botanist, Hugo de Vries. Other naturalists had 

 previously noted such aberrant or anomalous organisms but without 

 attaching much significance to them. Thus in the works of Darwin, 

 especially in "The Origin of Species" and "Animals and Plants under 

 Domestication," there are frequent references to aberrant individuals 

 or sports and to curious groups of plants and animals like the niata 

 cattle, which Darwin admits probably originated as definitely distinct 

 individuals among the typical species group. Yet Darwin never con- 

 sidered such aberrant individuals or groups as playing any significant 

 role in evolution. On the other hand, de Vries became so convinced of 

 the general occurrence and significance of suddenly appearing, heritable 

 variations that he proposed a theory of evolution by mutation in which 

 he applied Darwin's great principle of natural selection to these mutations 

 as the general method of origin of species. The investigations which 

 led him to this conviction extended over a period of nearly 20 years, dur- 

 ing which time he brought under experimental cultivation some hundred 

 species of plants that grow wild in Holland. They all exhibited more or 

 less continuous variation; also he was able to isolate numerous strains 

 which differed from the normal wild type with respect to some peculiar 

 feature. But de Vries was searching for evidence of species "in the 

 making" and he believed that by sufficient searching he should locate 



