Physiological Selection. 45 



although he did not publish until a year after the 

 appearance of my own paper in 1886*. 



I must next proceed to state some of the leading 

 features of physiological selection in further detail. 



It has already been shown that Darwin clearly 

 perceived that the very general occurrence of some 

 degree of infertility between allied species cannot 

 possibly be attributed to the direct agency of natural 

 selection. His explanation was that the slight struc- 

 tural modifications entailed by the transformation ol 

 one specific type into another, so react upon the 

 highly delicate reproductive system of the changing 

 type as to render it in some degree infertile with 

 its parent type. Now the theory of physiological 

 selection begins by traversing this view. It does 

 not, however, deny that in some cases the morpho- 

 logical may be the prior change ; but it strenuously 

 denies that this must be so in all cases. Indeed, 

 according to my statement in 1886, the theory inclines 

 to the view that, as a rule, the physiological change 

 is prior. At the same time, the theory, as I have 

 always stated it, maintains that it is immaterial whether, 

 "in the majority of instances,'* the physiological change 

 has been prior to the morphological, or vice versa ; 

 since in either case the physiological change will 

 equally make for divergence of character. 



' Zool. Journal, Lin. Soc, vol. xix. pp. 337-411 (1886); and for 

 Mr. Gulick's papers, ibid., vol. xx. pp. 1S9-274 (1887), vol. xxiii. 

 pp. 312-380 (1889). Mr. Gulick has recently drawn my attention, in 

 a private letter, to the fact that as early as 1872 a paper of his was 

 read at the British Association, bearing the title Diversity of Evolution 

 under one set of External Conditions, and that heie the principle of 

 physiological segregation is stated. Although it does not appear that 

 Mr. Gulick then appreciated the great importance of this principle, it 

 entitles him to claim priority. 



