THE UNKNOWN FACTORS OF EVOLUTION. 89 



direction. Nageli's factor, on the other hand, assumes definite 

 but not necessarily adaptive ' phylogenic ' variation — his views 

 have been very generally misconceived on these points — and, 

 as he pointed out, his factor would also require Selection to 

 determine which of the definite lines of growth were adaptive. 



It seems necessary to thus clearly state the relations of the 

 time stages of variation to each of the five factors, in order to 

 show the decisive bearings our future exact research will have 

 upon them. For example, the proof that variation is either 

 ' definite ' or that it is ' adaptive ' prior to or independently of 

 Selection, will constitute conclusive disproof not of Darwin's 

 theory but of Neo-Darwinism. The fate of Lamarckism, on 

 the other hand, depends upon the demonstration that phylogenic 

 variation is not only ' definite ' and ' adaptive ' but that it is 

 anticipated by corresponding somatogenic variation. 



A review of recent thought upon the variation problem 

 shows that these life stages are becoming generally recognized. 

 I shall pass by Lamarck's and Darwin's factors which are so 

 thoroughly understood and speak only of the other three. 



Buffon's Factor in Variation. 



As regards Buffon's factor, which is the most comprehensive 

 of all, we know that Spencer and Weismann both assumed that 

 the direct action of the environment was primarily a factor of 

 evolution. Weismann first regarded this solely as the proto- 

 zoan source of Variation, but has recently given it a wider 

 play in the action of environment upon the germ-cells as a 

 cause not of definite variation but of variability. The line 

 of research upon the dynamic action of environment in its 

 influence upon somatogenic variation followed by Hyatt, 

 Dall, and others, is paralleled in the more recent specula- 

 tion connecting the environment directly with the gonagenic 

 and gamogenic stages, initiated by Virchow,i Kolliker,^ 



1 R. Virchow : Descendenz und Pathologie. Virchow's Archiv, CIII, 1SS6, pp. 

 1-15, 205-215, 413-437. Ueber den Transformismus. Archiv f. Anthropologic, 

 1889, p. I. 



^Kolliker: Das Karyoplasma und die Vererbung. Zeitschr. f. wissenschaftl. 



