SEASONAL DIMORHISM. Ordinary sexual di is where there are own forms of one sex, the cause of such a modification being un known. In seasonal dimorphism, however, the cause is due to changes of temperature. by subjecting the chrysalids of lowland butter flies to prolonged cold in ice-chests Weisman!' proved that northern or alpine species are climatic or seasonal varieties. \V. II. Edwards has shown that two of the four polymorphic forms of Papilio Ajax ( i.e. ll'afshii and Teta monides) winter chrysalhls, and Papilio Alarcellus emerges from a second hrood of summer ehrvsalids.
Cases of seasonal dimorphism frequently oc cur among tropical butterflies. While in the north or temperate zone we have winter and sum mer forms, in the tropics of India and of Africa there are wet-season and dry-season forms. It is curious that the difference between the two forms principally consists in the fact that one looks on the under side like a dry leaf, while the other is marked with eye-like spots or oeelli. Their identity has been proved by raising both forms from the. same hatch of eggs. This ease appears to be due to the direct action of the season, in the dry form to dryness and heat, in the wet form to the moisture and coolness of the wet season. Weismann, however, maintains that these changes of climate or season are only the stimulus, not the actual causes,' the latter being the processes of selection, a quite hypothetical cause, although twenty years ago he attributed the change to the effect of change of temperature.
Limitations of space forbid one enlarging on this fascinating theme, but we may briefly refer to the remarkable experiments of Standfuss, who, by subjecting pupa of Swiss butterflies to heat or cold, has produced artificially true temperature varieties, as follows: ( I ) Seasonal forms, similar to those known in nature ( Vanessa cardui albus, and Papilio llachaon to some extent). (2) Lo cal forms and races similar to those which occur constantly in certain localities ( vrticw, cardui, and to some extent Papilio ilachaon and Vanessa Ai/Nom!). (3) Entirely exceptional forms or aberrations. also occurring from time to time in nature ( Vanessa .to. cardui. Argynnis, Aglaia). (4) Phylogenetic forms, 110t note ex isting, "but which may either have existed in past epochs or may perhaps be destined to arise in the future" ( Vanessa lo. Antiopa, Atalanta). The conclusion from this and experiments by others shows, as Standfuss claims, that such forms are the result of the direct action of a change in the temperature.