442 G id ick — Cross-infertil ity. 



pletely segregated by what I have called Potential Segregation. 

 In other words, if the pollen of each species is potent only 

 when falling on the stigma of the same species then the species 

 are completely segregated though growing in the same area. 

 Still further two species may be fairly fertile when artificially 

 ■crossed, and yet be completely segregated while growing to- 

 gether, through the fact that the pollen of either species when 

 falling on its own stigma will be prepotent over the pollen of 

 the other species even though the alien pollen has fallen upon 

 the stigma considerable earlier. This I have called Prepoten- 

 tial Segregation. ISTow a variety that is segregated from the 

 parent form by prepotential Segregation and cross-infertility, 

 will neither fail of propagating nor be swamped by crossing 

 though it is indiscriminately mingled with the parent form. 

 In my paper on " Divergent Evolution " I have referred to 

 this special combination of positive and negative segregation, 

 produced by the incompatibility of the male and female ele- 

 ments, and have endeavored to show that when these charac- 

 ters occur together, they tend to increase in intensity accor- 

 ding to a law of self-accumnlation. (Linn. Soc. Jour. Zoo!., voL 

 xx, pp. 239-40, 259-60). AYithout here entering into any 

 computation, it is evident that the prepotency of the pollen of 

 each kind with its own kind, if only very slight, will prevent 

 cross fertilization as effectually as a moderate degree of instinc- 

 tive preference in the case of an animal, and if segregate 

 fecundity, (i. e. cross-infertility) is added it will tend to keep 

 the variety from the swamping effect of the little crossing that 

 occurs, and the variations that are above the average in these 

 characters will have the largest influence on the pure form in 

 each successive generation. 



I regard Physiological Segregation as including all kinds of 

 incompatibility between the male and female elements of 

 different groups, whether these groups are varieties of one 

 species, or species of one genus, or species of different genera, 

 or species representing still more divergent groups; and I 

 maintain that the importance of this principle in the origin 

 and continuance of divergent groups, cannot be exaggerated 

 in the case of organisms whose fertilizing elements are freely 

 distributed by wind or water ; for in these cases the segregate 

 compatibility and cross incompatibility of the male and female 

 elements may be the means by which the prevention of free 

 crossing is secured, as well as the means by which the swamp- 

 ing effect of the crossing that occurs is prevented. There is, 

 it seems to me, strong reason to believe that this principle is a 

 leading factor in the segregation of multitudes of water ani- 

 mals, as well as in the segregation of species of plants, whether 

 terrestrial or aquatic ; and Mr. Romanes has rightly empha- 

 sized the imjDortance of investigations in this line. 



26 Concession, Osaka, Japan. 



