Whoever answers this question in the affirmative will have gone most of the way towards accepting, on merely antecedent grounds, the theory of physiological selection. And therefore it is that I have begun this statement of the theory by introducing it upon these grounds, thereby hoping to show how extremely simple--how almost self-evident--is the theory which it will now be my endeavour to substantiate. I may here add that the theory was foreshadowed by Mr. Belt in 1874[16], clearly enunciated in its main features by Mr. Catchpool in 1884[17], and very fully thought out by Mr. Gulick during a period of about fifteen years, although he did not publish until a year after the appearance of my own paper in 1886[18].
[18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and for Mr. Gulick's papers, _ibid._, vol. xx. pp. 189-274 (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 here 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.
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 structural modifications entailed by the transformation of 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 morphological 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.