440 Gulick — Cross-infertility. 



great with the new form as with the old, and escape from 

 competition ceases Under these circumstances either the mal- 

 adaptation or the infertility of the hybrids will be of the high- 

 est importance in preventing swamping. But, is it necessary 

 to suppose, as Mr. Wallace does, that the infertility will dis- 

 appear in cases where the hybrids are as well adapted as the 

 pure forms ? In such cases, during the preliminary period 

 when escape from competition is gained most fully by those 

 most fully segregated, there will naturally be a rapid accumu- 

 lation of all segregative endowments ; but, when that con- 

 dition ceases, there will still be a sufficient reason for the con- 

 tinuance, or even increase, of the cross-infertility in the fact 

 that more than half of each generation of the pure form will 

 be the descendants of those whose cross-infertility , and other 

 segregative endowments are above the average. This princi- 

 ple is one form of what I have called Self-Cumulative Segrega- 

 tion. 



This law of self-accumulation does not seem to apply to 

 cross-infertility (or to any other form of negative segregation) 

 that is not associated with positive segregation. J^or is it 

 quite clear that, when unassociated with negative segregation 

 it applies to positively segregating characters (such as social 

 and industrial instincts that lead animals of one kind to pair 

 together, and the prepotency of the pollen of a given kind on 

 the stigma of the same kind securing a similar result for 

 plants). When, however, characters producing positive but 

 incomplete segregation are associated with those producing 

 negative segregation, both classes of characters must tend to 

 increase till the segregation becomes pronounced. As soon as 

 this point is reached, the Reflex Selection, by which the differ- 

 ent portions of the species have been kept in harmonious rela- 

 tions with each other, is suspended, and there is nothing but 

 the force of heredity to hold them in correspondence ; but the 

 force of heredity, securing this correspondence, has itself been 

 created by the long continued Reflex Selection, and when this 

 is removed, it gradually fails, and divergences of all kinds 

 multiply, increasing the incompatibility of the two forms. 

 Thus arises diversity of habits, diversity of sexual and social 

 instincts, and diversity in the affinities of the male and female 

 elements; and in each respect this diversity tends toward the 

 point of complete incompatibility. 



Positive segregation diminishes the amount of crossing, and 

 negative segregation diminishes the swamping effect of cross- 

 ing when it occurs. jSTegative segregation may be of the fol- 

 lowing forms : (1) lack of fertility of first crosses and of the 

 hybrids, which I call Segregate Fecundity ; (2) lack of Vigor 

 in hybrids, which I call Segregate Vigor; (8) lack of adapta- 



