Note 286 — VII.

[Professor Meldola first called attention to the scattered instances of the kind here alluded to by Professor Weismann, in 1873: see ‘Proc. Zool. Soc.,’ 1873, p. 153. The author explains the relation of this ‘variable protective colouring’ to other protective appearances, and he is strongly of the opinion that the former as well as the latter is to be explained by the action of the ‘survival of the fittest.’

The validity of Dr. Weismann’s interpretation of these effects as due to adaptation, through the operation of natural selection, is conclusively proved by the following facts. The light reflected from green leaves becomes the stimulus for the production of dark brown pigment in those cases in which the leaves constitute the surroundings for many months. Under these circumstances the leaves of course become brown at a relatively early date, and protection is thus afforded for the remainder of the period, although the dark pigment is produced before the change in the colour of the leaf. Instances of this kind are seen in the colours of cocoons spun among leaves by certain lepidopterous larvae (see ‘Proc. Ent. Soc. Lond.,’ 1887, pp. l, li, and 1888, p. xxviii), the cocoons of the same species being of a creamy white colour when spun upon white paper.

Conversely, the light reflected from the same surfaces serves as the stimulus for withholding pigment in the cases alluded to by Dr. Weismann (larvae of R. Crataegata, &c.), in all of which the organism only remains in contact with the leaves while they are green, viz. at a time when the dark colour would be disadvantageous.

Hence precisely opposite effects are produced by the operation of the same force; the nature of the effect which actually follows in any case being solely determined by the advantage afforded to the organism.—E. B. P.]