Page:The Effect of External Influences upon Development.djvu/28

24 of the primary constituents of the germ produced by processes of selection,—in this instance by adaptation of the summer generation so as to render it similar to a protected species which flies about along with it (see Note Vl, p. 59).

Thus it would not be inconceivable that the caterpillars of a species which produces two generations in a year should have become adapted in respect of protective colouring to two different and alternating food-plants; and in this case too, the periodic change of colour would apparently depend on the direct influence of the summer and autumn climate, while really due to the presence of double primary constituents in the germ, which by some external influence—perhaps warmth or perhaps the quality of the light falling on the young caterpillar-would only undergo development alternately.

It is possible that the caterpillars of the North American butterfly Lycaena pseudargiolus offer such an instance. W. K. Edwards in his admirable work on the Butterflies of North America says that the caterpillars of the summer and autumn broods in this species are quite differently coloured, the former being white and well protected on the white flower-buds of their food-plant, Cimicifuga racemosa, while the latter are yellow-green or olive-green, and live on the flower-buds of a plant (Actinomeris squamosa) which bears yellow flowers and blooms later in the year. Whether the last-named colouration is also to be regarded as protective unfortunately cannot be determined from the available observations, as the case would have