Page:The Effect of External Influences upon Development.djvu/27

Rh that appear under the influence of temperature. Many years ago I made experiments 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 effects 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 whether 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 my mind. But now, by the united labours of many excellent observers, we know that mimicry is of a much commoner and more widespread 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 with 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 included 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 might 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