KNOWLEDGE OF THE MUTATUVG OENOTHERAS. 59 



It by no means follows, hovveyer, that mutations appear only after a mixture of germ- 

 plasms, and it is necessary to distinguish two types of mutations : (1) those following 

 an ancestral mixture of germ-plasms, and f2) those occurring in pure lines. 



The numerous and rapidly -accumulating cases of mutations, not only in Oenothera 

 but in many other genera of plants and animals, make it impossible to ignore this 

 factor as a method of species- formation, whatever its evolutionary significance may be. 

 But series of chance-wise mutations could scarcely result in the orderly phylogenics we 

 often see in the palseontological record ; nor are they adequate to account for the more 

 complex cases of adaptation, especially those involving inter-relationships between 

 various organisms, though they may be capable of explaining cases of " climatic 

 adaptation " in various species of a genus. 



Darwin, in his theory of natural selection, also usually assumed a change in the 

 climatic or biological environment of a species, which led ultimately to its modification. 

 But the effects of climatic and distributional vicissitudes in the earth's history are again 

 scarcely competent to explain the occurrence of long, orderly sequences of forms. The 

 only effect of natural selection which is experimentally proven is its conservatioe effect 

 in maintaining the species in its present equilibrium and at the maximum of its 

 efficiency in the struggle for existence. That natural selection can produce a modifica- 

 tional effect is, like the inheritance of acquired characters, at present experimentally 

 unproven. 



A conspectus is given, showing roughly the relationships of the Oenothera species 

 belonging to the group which contains the mutating forms. 



In Section III. are summarized my studies on the present distribution of the mutating 

 Oenotheras, and the history of their introduction from North America into European 



r 



F 



Botanic Gardens. They have now become naturalized in nearly all regions of the world. 

 The early introductions into Europe can be identified in many cases by means of figures, 

 descriptions, and specimens. Certain of these races appear to have resembled closely 

 O. Lamarchiana, but no early record can be positively identified with this form to the 

 exclusion of 0. grandiflot^a or (in one case) 0. biennis. 



In Section IV. the mutation phenomena in O. LamarcUana are considered in detail, 

 the fact being emphasized and proven by illustrations that, although the ranges of 

 variation of the various mutaiits frequently overlap in various characters, yet their 

 modal conditions are different for each mutant, and the mutants are therefore clearly 

 discontinuous in their origin from the parent form. When the mutants from O. La- 



marchiana appear, their characters are fully developed in the first individual, and there 

 is no evidence that subsequent selection from the offspring of a mutant has any effect in 



intensifying its characters. 



Several of the mutant types, notably O. gigas and 0. nanella, show a surprising 

 range of variability, particularly in leaf-characters. The Oenotheras are also 

 very sensitive to environmental changes, and the environment is a very important 

 element in interpreting the wide range of fluctuation often observed in physiological 



i2 



