896 OX THE SUPPOSED BOTANICAL PROOFS OF THE 



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 to that of the Thuya-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 knowledge 

 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 conditions. We must, however, include among the 

 external circumstances everything which is not germ-plasm. More- 

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

 it as an instance which, if we assume it to be correct, further illus- 

 trates my view's upon the phenomena presented by the T/iiya-shoot. 



The two other facts brought forward by Detmer as proofs of 

 the transforming power of external influences can be explained 

 in precisely the same manner. These instances are the fact 

 that T-ropaeohim when grown in moist air produces leaves with 

 anatomical characters different from those produced when the plant 

 is grown in dry air ; and the differences in the structure of the 

 leaves of many plants, according as they have been grown in the 

 sun or shade respectively. Such differences do not by any means 

 afford proof of the direct production of structural changes by means 

 of external influences. How would such an explanation be con- 

 sistent with the fact that the leaves are, in all these cases, changed 

 in a highly purposeful manner ? Or is it assumed that these organs 

 were so constituted froni the beginning, that they are compelled 

 to respond to external conditions by the production of useful 



1, 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 B. 

 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 dis- 

 advantageous. 



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.] 



