276 GENETICS 1N 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 réle in plant development than has been realized. 
The recent experiments of Loeb on Bryophyllum calycinun 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 
réle 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 
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