186 THE entomologist's record. 



" not look upon the reaction of tlie scales of the butterfly as an inten- 

 tional one," but this does not necessarily exclude the possibility that 

 the determinants of the scales are so arranged in advance by natural 

 selection that tliey should produce black under the influence of high 

 temperature. 



8. — a biological explanation of the seasonal dimorphism 

 exhibited by Vanessa levana and its summer form prorsa. — 

 The explanation of the learned Professor with regard to the seasonal 

 dimorphism of the well-known Vanessa levana and its form prorsa 

 is probably the most unsatisfactory of all the explanations in the 

 paper. Having determined that C. phloeas is affected by external 

 influences for wdiich it is not fltted in advance, and that the increased 

 darkening is the outward sign of this difference, he goes on to say : — 

 " But in other and to all appearance similar instances the relations 

 may be of a different nature, though at a glance it may be impossible to 

 definitely decide that this is tlie case. We must at any rate be careful 

 not to regard as necessarily accidental all the variations that appear 

 under the influence of temperature. Many years ago I made experi- 

 ments with the seasonally dimorphic butterfly Vanessa levana-prorsa, 

 and was able to prove that the two forms of one and the same species, 

 while very different in colour and pattern, owe their difference to the 

 effect of different degrees of warmth during the pupal stage : it is at 

 least possible to convert the summer generation into the spring form, 

 by lowering the temperature. Even at that time it appeared to me 

 doubtful wliether such a total change in colour and pattern in the 

 summer form of V. prorsa could actually depend only on the chance 

 influence of a higher temperature, and the idea of mimicry at first 

 crossed niy mind. But now, by the united labour.s of many excellent 

 observers, we know that mimicry is of a much commoner and more 

 wdde-spread occurrence than could formerly have been supposed, and I 

 should now consider it possible that the summer form — V. prorsa — 

 might have resulted from imitation of Limenitis sibylla, which flies Avith 

 it in clear spaces in the woods, and to which in fact it is strikingly 

 similar. I cannot, however, at present give a proof in support of this 

 supposition, and am not even able to say whether Limenitis is to be in- 

 cluded among protected species. The reasons which lead me to this 

 conclusion cannot be given here in detail, and I mention the idea only 

 as an illustration — whether real or imaginary — of how the impression 

 mio'ht arise that a metamorphosis was due to external influences, while 

 the influence— in this case warmth -had only to play the part of the 

 stimulus, the real cause being a variation of the jirimary constituents of 

 the germ, produced by processes of selection — in this instance by adapta- 

 tion of the summer generation so as to render it similar to a protected 

 species which flies about along with it." So far as Professor Weismaun 

 commits himself to an opinion here, it would lead one to sujjpose that 

 variation in the germ, followed by the external stimulus of a high 

 temperature in the pupal stage, leads the red-brown Vanessa levana 

 (coloured somewhat like a Tortoise-sliell butterfly) to assume the dark 

 coloration of Vanessa prorsa (coloured of a deep black-brown, and re- 

 sembling somewhat, to a superficial observer, Limenitis sibylla) in order 

 that it may mimic Limenitis sibylla. If one asks on wliat grounds Professor 

 Weismann is led to such conclusions, it would ajjpear that they are 



