DARWIN AND PALEONTOLOGY 



or factors of evolution. Subsequent research 

 has convinced me that in these phenomena of the 

 internal origin first of certain determinate char- 

 acters, and second of certain adaptive charac- 

 ters, we are deahng with the unknown if not with 

 the unknowable, although the latter despairing 

 attitude should not be hastily adopted. The 

 immediate causes are internal, that is, they lie in 

 the domain of heredity rather than of ontogeny, 

 environment, or selection; but lest I might be 

 mistaken on this point, I have devoted several 

 years of thought to the development of a circle 

 of causes J so to speak, which I have finally formu- 

 lated ^ in the law called the four inseparable fac- 

 tors of evolution. According to this law I re- 

 gard heredity not as something inseparable, al- 

 though extraordinarily stable; on the contrary I 

 have recently expressed the relations of heredity 

 to the other factors as follows : — 



The life and evolution of organisms contin- 

 uously center around the processes which we 

 term heredity^ ontogeny^ environment^ and se- 

 lection; these have been inseparable and inter- 

 acting from the beginning; a change introduced 

 or initiated through any one of these factors 

 causes a change in all. First, that while insep- 

 arable from the others, each process may in cer- 



' The Four Inseparable Factors of Evolution. Theory of 

 Their Distinct and Combined Action in the Transformation of 

 the Titanotheres, an Extinct Family of Hoofed Animals in the 

 Order Perissodactyla," Science, N. S., Vol. XXVII, No. 682, 

 January 34, 1908, pp. 148-50. 



