408 ON SUPPOSED BOTANICAL PROOFS OF [VII. 



of assuming a lighter or darker colour \ The light which falls 

 upon a single individual caterpillar during the course of its 

 growth determines whether the lighter or darker colour shall 

 be developed. Here therefore we have a case exactly parallel I 



to that of the Thuja-shoot in which the palisade or spongy " 



parenchyma is developed according to the position in which 

 the shoot is fixed. 



As far as it is possible in the present condition of our know- 

 ledge to offer any opinion upon the origin of sex in bisexual 

 animals, it may be suggested that this problem is also capable 

 of an essentially similar solution. Each germ-cell may possess 

 the possibility of developing in either of two directions, the one 

 resulting in a male individual, and the other resulting in a 

 female, while the decision as to which of the two possible 

 alternatives is actually taken may rest with the external con- 

 ditions. We must, however, include among the external 

 circumstances ever3^thing which is not germ-plasm. Moreover, 

 this explanation is by no means certain, and I only mention 



^ [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 prodiictiott 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 larx'ae tsee 

 'Proc. Ent. Soc. Lond.,' 1887, pp. 1, li, and 1888, p. xxviii, also 'Colours 

 of Animals,' pp. 142 146), 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 ivithholding pigment in the cases alluded to by Dr. Weismann 

 (lar\'ae of R. Crataegala, &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.J 



