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Show 1 9 0 4 .] PHASES in b u t t e r f l ie s . 1 4 3 to have some idea that because a type of pattern and coloration was characteristic of a particular season or climate, it did not necessarily exclude other types : therefore that it was not impossible for phases characteristic both of dry and wet seasons or climates to be sometimes found flying together; that in a very dry country like Aden it was the rule rather than the exception for wet, intermediate, and dry phases of a species to occur commonly together in each brood. That this polymorphic character was probably of earlier date than the more or less defined seasonal phases, of such countries as exhibit great variations of weather at different seasons, seemed evident to me from the fact that in very moist countries the extreme dry phase of species is exceedingly rare, and probably near to extinction. In Precis sescimus, the dry phase of P. natal-ensis (= calescens) from Southern and Eastern Africa, the seasonal phases are very distinct, but about equally abundant. In the wet season, as pointed out by me (P. Z. S. 1898, p. 904), both forms may be taken flying together in Mashunaland; and on that ground I proposed that the term " seasonal form " should be rejected, and the term " seasonal phase " substituted *. On the West Coast P. calescens or natalensis is represented by P. octavia of Cramer and a number of intermediate phases, but no extreme dry phase was recorded until 1901, when, in my " Revision of the Butterflies of the genus Precis " (Ann. &, Mag. Nat. Hist. ser. 7, vol. viii. p. 205), I mentioned an imperfect example indistinguishable from typical P. sesamus as having been received from Onitsha on the Niger. The extreme rarity of this phase on the West Coast, and the probability that it has become absolutely extinct at Sierra Leone, seem to indicate that it is unsuited to the conditions of a moist climate; whilst the numerous intergrades from the dry to the wet phase on the same coast certainly indicate the transition from fixed varieties, such as obtain where seasons are well defined, towards a more or less wet type. In Southern and Eastern Africa intergrades between P. sesamus and P. natalensis are extremely rare, the most striking of such intergrades being figured by me in 1900 (P. Z. S. pi. lviii. fig. 1). As it is by no means rare for individuals of the wet phase of a species to emerge from the pupa in the dry season, there is no reason why Lepidopterists should be startled when this occurs. They should bear in mind the probability of the fact that, as all the phases of some species occur as simple varieties in extremely dry countries, they also formerly existed as varieties in other species; that the latter, as they gradually extended their range, were subjected to widely different conditions; that then the summer phase (as we now understand it) was so conspicuous in the winter, and the winter phase so conspicuous in the summer, that their chance of survival at the unsuitable season was lessened ; and thus it came about in course of time that one variety of the species became the prevalent wet phase, and * See also P. Z. S. 1900, p. 916- |
