G ul ick — Cross-infertility. 441 



tion in hybrids, which I call Segregate Adaptation ; (4) lack 

 of escape from competition in hybrids, as compared with pure 

 forms, which I call Segregate Escape from Competition. Of 

 these all but the 4th were considered in my paper on " Diver- 

 gent Evolution through Cumulative Segregation," where I 

 endeavored to show that in their cooperation with positive 

 segregation they were parallel factors producing similar results. 

 JSow in his supposed case (pp. 173-9), Mr. Wallace has treated 

 the Segregate Adaptation, or hybrid maladaptation, as if it 

 were the effective factor by which the hybrid infertility 

 is alone enabled to increase or even continue. I see no reason 

 why this should be so. The effectiveness of these negative fac- 

 tors in preserving a species must depend on their being asso- 

 ciated with positive segregation ; but the effectiveness of any 

 one of the negative factors is not destroyed by the absence of 

 the others; though Segregate Escape from competition is, 

 under ordinary conditions, confined to the preliminary stages 

 of divergence ; and may, in certain cases, be the necessary con- 

 dition leading to the other forms of negative segregation. 



Mr. Wallace's criticism of the theory of physiological Selec- 

 tion (pp. 180-3) is unsatisfactory ; (1) because he has adopted 

 the fundamental principle of that theory, on pages 173-9, in 

 that he maintains that without the cross-infertility the incipi- 

 ent species there considered would be swamped ; (2) because 

 he assumes that physiological selection pertains simply to the 

 infertility of first crosses, and has nothing to do with the infer- 

 tility of mongrels and hybrids ; (3) because he assumes that 

 infertility between first crosses is of rare occurrence between 

 the species of the same genus, ignoring the fact that, in many 

 species of plants the pollen of the species is prepotent on the 

 stigma of the same species when it has to compete with the 

 pollen of other species of the same genus ; (4) because he not 

 only controverts Mr. Romanes' statement that cross-infertility 

 often affects " a whole race or strain," but he gratuitously 

 assumes that the theory of Physiological Selection excludes 

 this "racial incompatibility" which Mr. Romanes maintains 

 is the more probable form, and bases his computation on the 

 assumption that the cross-infertility is not associated with any 

 form of positive segregation ; (5) because he claims to show 

 that " all infertility not correlated with some useful variation 

 has a constant tendency to effect its own elimination, while his 

 computation only shows that, if the cross-infertility is not 

 associated with some form of positive segregation, it will dis- 

 appear ; and (6) because he does not observe that the positive 

 segregation may be secured by the very form of the physiolo- 

 gical incompatibility. Many species of plants may be pro- 

 miscuously distributed over the same area, and still be com- 



Am. Jour. Sci. — Third Series, Tol. XL, No. 240. — Dec, 1890. 

 28 



