( 444 ) 



Fmust soon be the result, if at the first appearance of Fthe number of individuals 

 of this variety was not very great. With every successive brood the percentage of 

 varietal specimens produced by F will become higher, and to make up for this we have 

 assumed that the very first specimens of V produced already 80 per cent, of the 

 varietal form. 



There remains now only the second alternative, that Kv as the descendant of V 

 produces both F and N, which agrees with what we know of the propagation of 

 varieties which occur among normal specimens, and hence is the onlj- acceptable 

 alternative. The offspring F^i of iVu crossing with Fwill bring the blood of iV\' and 

 N, which interbreed, into F, just as ^v (as the offspring of F) brings the blood of F 

 into iV, and this would go on as long as Fand iV^ exist together in the same locality. 

 Though N and Fare mutually sterile, the blood of N comes into Fby means of Vn, and 

 the blood of F into N by means of Nv ; this indirect intercrossing will completely 

 annihilate the efifect of the assumed mutual sterility of N and F. The following 

 diagram will serve to illustrate these lines : — 



It can easily be shown that after a certain time X and V will occur in equal 

 numbers.* 



The physiological selection will, therefore, in no case result in divarication of 

 a species into two, but the outcome of the physiological variation will be either 

 dimorphism of the species, when both the normal and the varietal form are equally 

 favoured in respect to the circumstances of life, or extinction of that form which is 

 the least favoured. If however the most favouralde kind of variation docs not lead 

 to the origin of a new species beside the parent one, no other variation will lead to 

 this end. Hence we must conclude that a divarication of a species into two or more 

 species cannot come about so long as the divergent varieties live so together that 

 a direct or indirect intercrossing is not jjreventcd. 



Having thus disposed of the possibility of the divarication of species 

 without the help of some kind of local separation, we have to consider the other 

 question : whether local separation as such can be able to give origin to a variety 

 and to transform a variety into a species. The theory of isolation as promoted 

 by Wagner says that the peculiar characters of some isolated sj)ecimens will by 

 breeding in and in finally be transmitted to all the descendants of those specimens, 

 and their degree of divergency become in the course of time so much higher that 

 these descendants represent a new species. 



Experiments teach us that aberrant specimens of a species occurring amongst 

 the normal specimens produce, when crossed together, ofl'spring which jjartly are of 

 the normal, partly of the varietal form of the species ; from black specimens of the 

 moths Ampliidasis betularius and Lijjaris monacha are obtained both black and 

 white individuals. To make the circumstances most favourable for the eventual 



.Mui|)hy, Habit and JiiUlliyeiiCf, Loudon, IU'V. p. 241. 



