196 EEV. J. T. GTJLICK ON DIVERGENT EVOLUTION 



complex environment ; it is therefore with special pleasure tliat 

 I observe that a law of very similar import may be derived by a 

 wholly different method from the general laws of action and 

 reaction in the physical world. It should, however, be noticed 

 that in the brief references made to the subject in Spencer's 

 ' Principles of Biology ' * it is assumed that " Increasingly- 

 definite distinctions among variations are produced wherever 

 there occur definitely-distinguished sets of conditions to which 

 the varieties are respectively subject," and only where these 

 occur ; for " Vital actions remain constant so long as the external 

 actions to which they correspond remain constant ; " and no 

 reference is anywhere made to the principle that whatever causes 

 sexual separation between dissimilar members of one family, race, 

 or species tends not only to perpetuate, but to increase their 

 dissimilarity in the succeeding generations. The view maintained 

 in the following paper is, I believe, in better accord with the 

 fundamental principle that " Unlike units of an aggregate are 

 sorted into their kinds and parted when uniformly subject to the 

 same incident forces," t as is also the teaching of Spencer's 

 ' Principles of Biology ' in one passage ; for I have recently dis- 

 covered that in a single paragraph of this work it is maintained 

 that, while exposed to the same external conditions, the members 

 of the same species may be increasingly differentiated, " until at 

 length the divergence of constitutions and modes of life become 

 great enough to lead to segregation of the varieties." + If 

 the segregation had been introduced as a necessary condition 

 without which the divergence of families and races could not 

 take place, the position taken in this paragraph would have been 

 essentially the same as the one I have adopted. In the next 

 section, however, he abandons the position, using the following 

 words : — " Through the process of differentiation and integration 

 which of necessity brings together, or keeps together, like indi- 

 viduals, and separates unlike ones from them, there must never- 

 theless he maintained a tolerably uniform species^ so long as there 

 continues a tolerably uniform set of conditions in which it may 

 exist y [The italics are mine.] 



I trust my endeavour to contribute something toward the 

 development of the theory of divergent evolution will not be 



* Compare §§ 91, 156, 169, 170. 



t See Spencer's ' First Principles,' § 166, near the end ; also a fuller state- 

 ment in § 169. \ See § 90. 



