44 Studies in the Theory of Descent. 



than the winter form of the lowlands, because it is 

 the older, and should therefore be the more con- 

 stant in its characters. It must not be forgotten, 

 however, that the variability of a species may not 

 only originate in the one familiar manner of un- 

 equal response of the individual to the action of 

 varying exciting causes, but also by the crossing of 

 two varieties separately established in adjacent 

 districts and subsequently brought into contact. 

 In the Alps and Jura the ordinary form of Napi 

 swarms everywhere from the plains towards the 

 habitats of Bryonice, so that a crossing of the two 

 forms may occasionally, or even frequently, take 

 place ; and it is not astonishing if in some places 

 (Meiringen, for example) a perfect series of inter- 

 mediate forms between Napi and Bryonia is met 

 with. That crossing is the cause of the great 

 variability of Bryonicz in the Alpine districts, is 

 proved by the fact that in the Polar regions this 

 form " is by no means so variable as in the Alps, 

 but, judging from about forty to fifty Norwegian 

 specimens, is rather constant." My friend, Dr. 

 Staudinger, who has twice spent the summer in 

 Lapland, thus writes in reply to my question. 

 A crossing with Napi cannot there take place, 

 as this form is never met with, so that the ancient 

 parent-form Bryonia has been able to preserve its 

 original constancy. In this case also the facts 

 thus accord with the requirements of the theory. 



