172 Fhenomena of Variation and Geographical Distribution. 



mediate region both black and yellow females occur in varying 

 proportions. Lat. 37° is approximately the southern limit of the 

 yellow form, and 42° the northern limit of the black form ; and, to 

 render the proof complete, both black and yellow insects have been 

 bred from a single batch of eggs. He further states, that out of thou- 

 sands of specimens he has never seen or heard of intermediate varieties 

 between these forms. In this interesting example we see the effects of 

 latitude in determining the proportions in which the individuals of each 

 form should exist. The conditions are here favorable to the one form, 

 there to the other ; but we are by no means to suppose that these con- 

 ditions consist in climate alone. It is highly probable that the existence 

 of enemies, and of competing forms of life, may be the main deter- 

 mining influences ; and it is much to be wished, that such a competent 

 observer as Mr. Walsh would endeavour to ascertain what are the 

 adverse causes which are most efficient in keeping down the numbers 

 of each of these contrasted forms. 



Dimorphism of this kind in the animal kingdom does not seem to 

 have any direct relations to the reproductive powers, as Mr. Darwin 

 has shown to be the casein plants, nor does it appear to be very general. 

 One other case only is known to me in another family of my eastern 

 Lepidoptera, the Pieridce ; and but few occur in the Lepidoptera of 

 other countries. The spring and autumn broods of some European 

 species differ very remarkably ; and this must be considered as a 

 phenomenon of an analogous though not of an identical nature. 

 Araschnia prorsa, Central Europe, is a striking example of this alternate 

 or seasonal dipmorphism, Mr. Pascoe has pointed out two forms of 

 the male sex in some species of Coleoptera belonging to the family 

 Anthribidaa, in seven species of the two genera Xenoceriis and Mecocerus 

 (Proc. Ent. Soc. Lond., 1862, p. 71) ; and no less than six European 

 Water-beetles, of the genus Dytiscus, have females of two forms, the 

 most common having the elytra deeply sulcate, the rarer smooth as in 

 the males. The three, and sometimes four or more, forms under which 

 many Hymenopterous insects (especially ants) occur, must be con- 

 sidered as a related phenomenon, though here each form is specialized 

 to a distinct function in the economy of the species. Among the 

 higher animals, albiuoism may, as I have already stated, be considered 

 as analogous facts, and I met with one case of a bird, a species of 

 Lory (^Eos fuscata, Blyth), clearly existing under two forms, since I ob- 

 tained both sexes of each from a single flock. 



The fact of the two sexes of one species differing very considerably 

 is so common, that it attracted but little attention till Mr. Darwin 

 showed how it could in many cases be explained by what he termed 



