420 The Living Plant 



which happen to be adaptive would be preserved in the struggle 

 for existence, while the unadapted would perish. Then, according 

 to the theory, after a time the surviving species would mutate 

 again, and the fittest of its mutants would survive, and so on. 

 Thus would the species be kept adapted approximately to en- 

 vironments, while evolution would take place in a series of short 

 abrupt steps separated by long pauses, — a condition illustrated 

 in the lower part of our diagram (figure 172). De Vries' view of 

 evolution has, moreover, one marked advantage over Darwin's 

 in explaining the existence of species and structures which bear 

 no adaptive relations to the environment, such for example as 

 the wonderfully diverse forms and markings of the Diatoms ; for 

 such features can originate and reach a considerable degree of 

 development by mutation, if not cut off by natural selection, 

 while on Darwin's view only those things which were adaptive 

 had any chance of a considerable development. 



The great interest of de Vries' work at once stimulated a wide 

 search for other examples of mutation in both animals and plants ; 

 and a very few other cases have been discovered. Attempts have 

 also been made to set species into mutation experimentally, but 

 with only dubious success. It is thus plain that species in muta- 

 tion are very few, — which fact is explained by de Vries on the sup- 

 position that species display short periods of mutation separated 

 by long periods of quiescence. 



But though species in mutation are undoubtedly rare, species 

 apparently identical with mutants have been found to be common, 

 though there is no evidence as to how they have arisen. The more 

 intensive studies of the past few years have shown that species 

 formerly considered single are in reality aggregates of dozens or 

 hundreds of these mutant-like species, which have now become 

 so prominent in scientific literature that they have attained to 

 a distinctive designation of their own, viz., elemeMary species or 

 hiotypes. Thus the little ''Ladies Tobacco" of our earliest spring 

 fields, thought by the acutest observers of the last generation to 



