From time to time records appear of individual specimens more or less fulvous being caught in southern England, especially in the New Forest.[13] It would be interesting to know what offspring such individuals might produce. From the evidence now given some notion both of the strength and the weakness of the case considered as one of continuous climatic variation can be formed. I know no other equally satisfactory. Whether or not definite mixture of the intermediates with either of the extremes will be proved to occur, the case differs materially from those considered in the last chapter in the fact that at all events there is no general overlapping of forms. In a species so little given to wandering, overlapping could indeed scarcely be expected to occur. It is this circumstance which makes the species preeminently suitable as a subject for the study of climatic influences, and I trust that entomologists with the right opportunities may be disposed to explore the facts further. Just as many species, like _egeria_, have varieties which can be regarded as adapted to northern and southern regions, so there are also several which have lowland and Alpine forms quite distinct from each other. Every such case presents an example of the problem we have been considering. As the collector passes from the plains to the Alpine region, how will he find the transition from one form to the other effected? Does the lowland 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 complications 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 abundance. 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.