KNOWLEDGE OP THE MUTATING OENOTHEEAS. 7 



related forms they contain. It seems evident that in many cases the difPerences 

 between these elementary species (using the term in DeVries's sense) are not of value 

 for selection. -The limits of natural selection as an eliminating factor are probahly 

 much more circumscribed than extreme selectionists have supposed. 



If we refer briefly to other factors which are evidently concerned in species-origin 

 and evolutioUj we must compare mutation, as an evolutionary factor, with natural 

 selection. If there is any one point in Darwinian natural selection which has not 

 been fully appreciated in post-Darwinian discussions, it is, perhaps, that Darwin 

 assumed, at least in most cases, an original environmental change for the organism, 

 which led to the necessity for its readjustment to its environment, modification of the 

 species being produced during this period of readjustment. Thus the change in the 

 climate of a given area or the introduction of a sj^ecies into a new area with different 

 environmental conditions are continually referred to by Darwin as the initial changes 

 leading to modification of particular species. When no environmental changes were 

 taking place, natural selection was looked upon as maintaining the nicely-balanced 

 equilibrium between the species. It is always a climatic change, or the entrance of 

 new organisms, which disturbs that equilibrium, and natural selection then becomes a 

 factor producing modification. True, Darwin often emphasizes the keenness of the 

 struggle for existence, and its effect in bringing and maintaining all the organs of the 

 competing organisms at their highest degree of efficiency. But when some new 

 departure is to be made in the structure of any organism, a primary environmental 

 change is first supposed to throw the organism to one side of the centre of its 

 equilibrium, this being followed by progressive change through the effects of the 

 selection of variations, until the organism is once more adjusted to the mean of its 

 organic and inorganic environment. 



Two distinct effects of natural selection were therefore recognized by Darwin: 



1 



(1) a conservative effect, maintaining the race at its highest degree of efficiency while 

 the environmental conditions remain unchanged ; and (2) a modificational effect, 

 causing the selection of new variations and the final estabUshment of a new equilibrium 

 when the previous equilibrium has been disturbed by an organic or an inorganic 



environmental change. 



Darwin's distinction between these two independent effects of natural selection is by 

 no means always clear. But recent experimental work, particularly, has emphasized 

 the importance of natural selection as a conservatiFe force, maintaining the condition of 

 equilibrium which exists between any organism and its whole environment. It is a 

 condition to which organisms have been almost universally subjected during the whole 

 period of evolution. But it is not a force, and it is not as constant and unintermittent 

 in its effect, as, e. g., gravitation. The modificational effect of selection has, on the other 

 hand, thus far remained unproven, notwithstanding the experimental researches of the 

 last decade. As a result of the negative evidence from experimental work, W(i are now 

 in much the same position with regard to both selection and the inheritance of 

 acquired modifications. They are still unproven, though there is a certain amount of 



