ON DYNAMIC INFLUENCES IN EVOLUTION. 



By Wm. H. Dall.* 



It is generally admitted that in the doctrine of Natural 

 Selection we have a theory which accounts for the perpetuation 

 of favorable variations in organic beings and their progeny, 

 and for the elimination in the long run of those which vary in 

 unfavorable directions. It is equally admitted that the origin 

 of variation is not accounted for by this theory. In order to 

 round out our conception of the mode of evolution of the or- 

 ganic universe it is necessary that this deficiency should be 

 supplied, and that to it should be added some conception of the 

 mode by which variation is sustained in any given direction 

 until it has reached a point where its usefulness is sufficiently 

 marked to enable the selective process to operate. Besides 

 this it is hardly doubtful that there are many characters devel- 

 oped in organisms, especially those of the lower rank, in which 

 selection of an}- sort is but little concerned. 



It is not necessar}' to recapitulate the names of those who 

 have turned to the relations between the organism and its 

 environment as the onty nidus of the influences sought. Such 

 an enumeration would comprise nearly all American biologists 

 of prominence and many foreign naturalists. 



On the other side of the Atlantic a small but not unimpor- 

 tant number of biologists, of whom Weismann and Lankester 

 may be taken as spokesmen, have recently endeavored to show 



*Read before the Biological Society of Washington, Mar. 8, 1890. 



