174 PROBLEMS OF GENETICS 



transition from one form to the other effected? Does the low- 

 land form give place to the Alpine form suddenly, with a region 

 in which the two are mixed, or will he find a zone inhabited by 

 an intermediate population? I have spent a good deal of time 

 examining the facts in the case of Pieris napi and its Alpine 

 female variety bryoniae, and though there are many compli- 

 cations which still have to be cleared up, no doubt is possible 

 as to the main lines of the answer. If in any valley in the Alps 

 inhabited by both napi and bryoniae the collector catches every 

 specimen he can, beginning at the bottom and working up to 

 7,000 feet, he will at first get nothing but napi. At about 2,500 

 feet, he may catch an occasional bryoniae flying with the napi. 

 After 3,000 feet napi usually ceases, and only bryoniae are found. 

 As an exception a colony of napi may be met with at much 

 greater heights. I once found them in numbers at about 6,000 

 feet. 14 Not only were they free from any trace of modification 

 in the direction of bryoniae, but they were of the thoroughly 

 southern type of napi, being a late brood of that large and very 

 pale kind (meridionalis) almost destitute both of dark veining 

 above and of green veining below, which are common on the 

 shores of Lago Maggiore and in other hot southern localities. 

 Not far off at the same level were typical bryoniae in fair abund- 

 ance. Occasionally an intermediate may be met with. I have 

 taken a few, for example, at Macugnaga and at Fobello. These, 

 however, in my experience are rarities in the Alps. Fleck 15 

 gives notes on the distribution in Roumania which shows the 

 same state of things. The lowland form is not transformed 

 though found at great heights, and at Azuga (nearly 3,000 feet) 

 bryoniae occurs with only occasional "flavescens," viz., inter- 

 mediates of the second brood. 



If this were all the evidence we should be satisfied that the 

 lowland and Alpine types keep practically distinct, overlapping 

 occasionally, but rarely interbreeding. The problem would 

 remain, how is the distinctness of the two types maintained in 

 the region of overlapping? Nowadays, I suppose, we should 



14 Above the Tosa falls. 



15 Bui. Soc. Sciinte, VIII, 1899, p. 691. 



