OTHER THEORIES OF SPECIES-FORMING. 249 



would be produced ; or if it were correlated with both size 

 and colour, varieties differing in respect of both characters 

 might be produced. As a matter of fact, this insect does 

 actually occur as four distinct varieties differing in colour, 

 form, and size, though whether in consequence of the oper- 

 ation of reproductive divergence it is of course impossible 

 to say." But this theory has been strongly criticised by 

 Karl Jordan, 22 who believes himself able to show that re- 

 productive divergence would not work in the way conceived 

 by Vernon, but actually in such a way as to establish an 

 intermediate form. 



Karl Pearson 23 has formulated a theory called "reproduc- 

 tive selection" which he believes to be distinct from both the 

 Romanes and the Vernon theories, and to which 



Xc&rson s 



theory of repro- he attributes an importance in evolution "equi- 

 ductive selection, potent to natura i selection, if indeed it be not 



prepotent." The theory is based on correlations which seem 

 to exist between the variation in some particular organ and 

 fertility. From studies of variation of height in 4,000 Anglo- 

 Saxon families and 1,182 Danish families, Pearson finds 

 that there exists a distinct correlation between fertility and 

 height in the mothers of daughters in these families. The 

 effect of this correlation is to render women less variable 

 and to raise their mean height. The quantities are small, 

 but are sufficient, if unchecked by natural selection, to raise 

 the mean height of women in forty generations by 3 1-4 

 inches. "A factor which would alter stature by about three 

 inches in 1,000 years is clearly capable of producing very 

 considerable results in the long periods during which evolu- 

 tion may be supposed to have been at work." 



Of large importance in any consideration of the relations 



(Click's im- of isolation to species-forming are the observa- 



portant observa- tions and conclusions of Gulick. Derived origi- 



tions and conclu- . 



aions, nallv from an exhaustive study of the variation 



J 



and life-conditions of certain land shells (Achatinellidae) 



