TRANSMISSION OF ACQUIRED CHARACTERS. 395 



many experiments hatched from the same batch of eggs) were 

 similarly exposed to the green leaves of the same food-plant, they 

 did not indeed become bright green like the leaves, but were in- 

 variably of a much lighter colour than the other larvae, while 

 many of them gained a brownish-green tint. The larvae of 

 Smerinthus ocellatus^ also possess the power of assuming different 

 shades of green and of thus approaching, to some extent, the green 

 of the plant upon which they happen to live. It is quite impossible 

 to explain the phyletic development of the green colour of these 

 and other caterpillars as due to the direct action upon the skin of 

 the green light reflected from the leaves upon which they sit. 

 The impossibility of such an effect was pointed out long ago by 

 Darwin, and also followed from my own investigations. Here, as 

 in the other cases, the only possible solution is afforded by natural 

 selection. The colour of the caterpillars has become gradually more 

 and more perfectly adapted to the colour of the leaves, and often 

 to the particular side of the leaves upon which these animals rest, 

 not by the direct effect of reflected light, but by the selection of 

 those individuals which were best protected. Poulton's experi- 

 ments quoted above prove that certain species which occur upon 

 different plants with different colours (or even in some cases upon 

 the differently coloured parts of the same plant), present us with 

 a further complication in the process of adaptation, inasmuch as 

 each individual has acquired the power of assuming a lighter or 

 darker colour 2 . The light which falls upon a single individual 



[* See the editorial notes by Raphael Meldola, in his translation of Weismann's 

 ' Studies in the Theory of Descent ' (the Essay on ' The Origin of the Markings of 

 Caterpillars,' pp. 241 and 306): also E. B. Poulton, in 'Proc. Roy. Soc.,' vol. 

 xxxviii. pp. 296-314; and in 'Proc. Roy. Soc.,' vol. xl. p. 135. E. B. P.] 



[ 2 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- J 53- 

 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 adapta- 

 tion, through the operation of natural selection, is conclusively proved by the follow- 

 ing facts. The light reflected from green leaves becomes the stimulus for the 

 production of dark broivn 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. 



