But, if it be true that physiological isolation between genetically allied groups must usually itself have been the product of a gradual evolution; and if, when fully evolved, it constitutes a condition of the first importance to any further differentiation of these groups (by preventing fusion again into one group, more or less resembling the original parent form), do we not perceive at least a strong probability that in the lower stages of its evolution such mutual infertility must have acted as a segregating influence between the diverging types, in a degree proportional to its own development? The importance of mutual sterility as a condition to divergent evolution is not denied, _when this sterility is already present in an absolute degree_; and we have just seen that, before it can have attained to this absolute degree _it must presumably, and as a rule, itself have been the subject of a gradual development_. Does it not therefore become, on merely antecedent grounds, in a high degree probable, that from the moment of its inception this isolating agency must have played the part of a segregating cause, in a degree proportional to that of its completeness as a physiological character? 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]. [16] _Nicaragua_, p. 207. [17] _Nature_, vol. xxxi. p. 4. [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.