[259] Clausen, H., _Landwirtschaftl. Jahrb._, 1890, xix., 893.
[260] Matthaei, G. L. C., _Trans. Philosoph. Soc._, 1904, cxcvii., 47; Blackman, F. F., _Ann. of Bot._, 1905, xix., 281.
If we wish to get a clear understanding of the causes of variation in the colour and pattern of butterflies, we must direct our attention to the experiments of Fischer, who worked with more extreme temperatures than his predecessors, and found that almost identical aberrations of colour could be produced by both extremely high and extremely low temperatures. This can be seen clearly from the following tabulated results of his observations. At the head of each column the reader finds the temperature to which Fischer submitted the pupæ, and in the vertical column below are found the varieties that were produced. In the vertical column A are given the normal forms:
------------+-------------+-------------+--------------+-------------+------------ 0° to | 0° to | A | +35° to | +36° to | +42° to -20°C. | +10°C. | (Normal | +37°C. | +41°C. | +46°C. | | Forms) | | | ------------+-------------+-------------+--------------+-------------+------------ _ichnusoides|_polaris_ |_urticæ_ |_ichnusa_ |_polaris_ |_ichnusoides (nigrita)_| | | | | (nigrita)_ _antigone |_fischeri_ |_io_ | ---- |_fischeri_ |_antigone (iokaste)_| | | | | (iokaste)_ _testudo_ |_dixeyi_ |_polychloros_|_erythromelas_|_dixeyi_ |_testudo_ _hygiæa_ |_artemis_ |_antiopa_ |_epione_ |_artemis_ |_hygiæa_ _elymi_ |_wiskotti_ |_cardui_ | ---- |_wiskotti_ |_elymi_ _klymene_ |_merrifieldi_|_atalanta_ | ---- |_merrifieldi_|_klymene_ _weismanni_ |_porima_ |_prorsa_ | ---- |_porima_ |_weismanni_ ------------+-------------+-------------+--------------+-------------+------------
The reader will notice that the aberrations produced at a very low temperature (from 0° to -20°C.) are absolutely identical with the aberrations produced by exposing the pupæ to extremely high temperatures (42° to 46°C.). Moreover, the aberrations produced by a moderately low temperature (from 0° to 10°C.) are identical with the aberrations produced by a moderately high temperature (36° to 41°C.).
From these observations Fischer concludes that it is erroneous to speak of a specific effect of high and of low temperatures, but that there must be a common cause for the aberration found at the high as well as at the low temperature limits. This cause he seems to find in the inhibiting effects of extreme temperatures upon development.