Opinions on Isolation. 115 



cipal defects, in my judgement, were, first, the in- 

 ordinately speculative character of some of its parts, 

 and, second, the restriction of its analysis to but one 

 form of isolation a defect which it shares with the 

 essays of Wagner, and in quite as high a degree. 

 Furthermore, although this essay had the great merit 

 of enunciating the principle of Amixia, it did so in 

 a very inefficient manner. For not only was this 

 principle adduced with exclusive reference to geo- 

 graphical isolation, but even in regard to this one 

 kind of isolation it was presented in a highly in- 

 consistent manner, as I will now endeavour to show. 



Weismann was led to perceive the principle in 

 question by the consideration that new specific char- 

 acters, when they first appear, do not all appear 

 together in the same individuals: they appear one 

 in one individual, another in another, a third in a 

 third, &c. ; and it is only in the course of succes- 

 sive generations that they all become blended in 

 the same individuals by free intercrossing. Hence, the 

 eventually emerging constant or specific type is the 

 resultant of all the transitory or varietal types, when 

 these have been fused together by intercrossing. 

 From which Weismann deduces what he considers 

 a general law- namely, that " the constancy of a 

 specific type does not arise suddenly, but gradually; 

 and it is established by the promiscuous crossing 

 of all individuals 1 ." From which again it follows, 

 that this constancy must cease so soon as the condition 

 which maintains it ceases i. e. so soon as free inter- 

 crossing is prevented by the geographical isolation 

 of a portion of the species from its parent stock. 

 1 Loc. cit., p. 43. 

 I a 



