[13] For these facts I am indebted to Mr. W. L. W. Field, who has lately published an account of his observations and experiments. See especially, _Psyche_, 1910, XVII, No. 3, where full references to previous publications are given. [14] For the facts and further references see W. H. Edwards, _Butterflies of N. America_, 2d series, Papilio VII and X; 3d series, 1897, Papilio IV, _Can. Entom._, 1895, XXVII, p. 239. [15] I think this case is fairly included because the _machaon_ type is so widespread that it cannot be regarded as a product of a Northern climate, nor can _asterias_ be claimed as especially a warm country form, seeing that _brevicauda_, which is scarcely distinguishable from _asterias_, inhabits Newfoundland (having a curious phase there in which the yellow is largely replaced by red). CHAPTER VIII LOCALLY DIFFERENTIATED FORMS. _Continued._ CLIMATIC VARIETIES In this chapter we will examine certain cases which illustrate phenomena comparable with those just considered, though as I have already indicated, they form to some extent a special group. The outstanding fact that emerges prominently from the study of the local forms is that when two definite types, nearly allied, and capable of interbreeding with production of fertile offspring, meet together in the region where their distributions overlap, though intergrades are habitually found, there is no normally or uniformly intermediate population occupying the area of intergradation. Such phenomena as these must, I think, be admitted to have great weight in any attempt to construct a theory of evolution. True we must hesitate in asserting their positive significance, but I see no escape from the conclusion that they throw grave doubt on conventional views. Again and again the same question presents itself. If _A_ and _B_ lately emerged from a common form why is that common form so utterly lost that it does not even maintain itself in the region of overlapping? Almost equally difficult is it, in the cases which I have numerated, to apply concrete suggestions based on any factorial scheme. We may see that in _Heliconius erato_ the type with the red mark on the hind wing probably contains a dominant factor, and that where the red mark is absent the metallic colours are exposed; and that similarly the green metallic colour may have another factor which distinguishes it from the blue. In this way we can fairly easily represent the various types of _erato_ on a factorial system as the result of the various possible combinations of two pairs of factors. But there we stop, and we are quite unable to suggest any reason why one area should have the red and the green type while another should have the blue also. So again with _Colaptes_ or the Warblers. By application of a factorial system, admittedly in a somewhat lax fashion, the genetic interrelations of the types can be represented; but how it comes