HYBRIDIZATION. 



151 



as owing their darker coloration to the cross, for it is well known 

 that A. betularia, like P. monacha, is undergoing a gradually in- 

 creasing melanism, which is probably protective, in many parts 

 of its area of distribution. The extreme aberration double- 

 dayaria, which thirty years ago was known only from Great 

 Britain, has now appeared in Westphalia, the Rhine Provinces, 

 Hanover, Gotha, and lastly in Dresden and Silesia. In several 

 of these places it is becoming more and more common, and in 

 at least some of them it is found side by side with the darkening 

 forms of A. betularia, which, though of different nature and 

 origin from the sport doubledayaria, are no doubt being preserved 

 and brought up to its level (in aspect) under the influence of 

 natural selection. 



" Boarmia repandata, L., ? , and ditto ab. conversaria, Hb., 

 ? . A large brood raised from the eggs of a pair of normal 

 B. repandata contained three males and one female of the 

 aberration conversaria. This female, which was paired with a 

 wild male B. repandata, produced twenty- eight repandata (of 

 which ten were males and eighteen females) and six conversaria 

 (four being males and two females). The majority of the larvse 

 died during the winter. Here again intermediate forms were 

 entirely absent. 



" From the above experiments in the pairing of normal 

 forms with aberrations and local races, performed or recorded by 

 Standfuss, he arrives at the following conclusions : — 



" 1. When the normal form of a species (Grimdart) is crossed 

 with a gradually formed local race of the same species, the result is 

 a series of intermediate forms. 



" 2. When the normal form is crossed with a sporadic aberra- 

 tion, the result in many cases is that the issue divides itself sharply 

 between the normal form and the sport, intermediate forms being 

 absent. 



"Hence, according to Standfuss, the process of species- 

 formation must be gradual ; for when two distinct species are 

 crossed, the issue does not split up into the two parental forms 

 as in the case when one parent is a suddenly formed aberration. 

 On the contrary, the behaviour of the issue of two distinct 

 species is very similar in kind to that of a species crossed with a 

 local race or variety which is being gradually established by the 

 accumulation of slight changes. It would seem therefore that 

 although an aberration or sport may be perpetuated by inherit- 

 ance, it can never acquire distinct specific rank. No doubt, 

 however, it may, if selected, eventually replace the original form 

 of the species." 



2p 



