196 COLOR PHOTOGRAPHY. 



Without contradicting this action, Semper 1 has lately maintained 

 that this explanation is not complete, and that, for example, the first 

 appearance of coloring matter in the covering of an animal is unex- 

 plained. This remark can, of course, not refer to colors which are to 

 be regarded as the insignificant characteristic of the chemical com- 

 pounds produced by the organization. It has, on the other hand, ref- 

 erence to the general lack of color observed in animals which live in 

 tlie dark. 



Semper 2 and Eimer 3 remark that the change in forms of life which 

 lies at the basis of Darwin's doctrine was taken by him simply as a 

 fact, and that it still lacks detailed explanation. Eimer 4 regards as 

 the causes of these variations the physical and chemical changes which 

 are brought about by the action of exterior conditions on living beings. 

 He attributes to the action of light a considerable influence on the 

 formation and alteration of the colors of animals. 5 



In such considerations one enters the domain of physical conceptions, 

 for such demand the regular procedure of an event with the simul- 

 taneously changing conditions. In contrast to a mechanical explana- 

 tion of this sort those of Darwin are to be distinguished as statical, and 

 take somewhat the same relative position with regard to it that the 

 explanation of the gas law by the kinetic theory of gases bears to the 

 purely mechanical explanation of the motion of the separate molecules. 

 The standpoint of observation in the two cases is, however, different. 

 For gases we consider a phenomenon as a whole, while in nature it is 

 generally the single items. I go into these general observations to 

 show that the two kinds of explanation do not exclude each other, but 

 on the other hand are complimentary. 



In this connection the establishment of the direct action of light on 

 the colors of animals deserves particular attention. Such an action 

 has been thoroughly investigated for caterpillars and butterfly pupae. 

 It was discovered by T. W. Wood 6 in the year 1867. The caterpillars 

 inclosed in chrysalises were brought into the sunshine and surrounded 

 by colored substances. They took the color of these surroundings. 

 How extensive this receptivity of pupae and caterpillars is has lately 

 been shown by the extraordinarily thorough and careful experimental 

 investigations of Edward B. Poulton. 7 



1 Semper, loc. cit., page 122. 



2 Semper, loc. cit., preface. 



3 Eimer, Entstehung der Arten, 1: page 1. 



4 Loc. cit., page 24. 



5 Loc. cit., pages 93, 145, 167, et al. 



r 'T. W.Wood. Proc. Ent. Soc, pages 99-101. 1867. Cited by E. B. Poulton, "The 

 Colours of Animals," London, Kegan Paul, Trench, Triibner and Co., 1890, who 

 himself has described the history of the discovery, page 113 ff. See also Poulton, 

 Phil. Trans., London, 178 : page 312. 1887. 



7 See besides the above-mentioned writings the comprehensive treatise, "Further 

 experiments upon the colour relation between certain lepidopterons larv.-e, pupae, 

 cocoons, and imagines and their surroundings." Transactions of the Entomological 

 Society of London, page 293. 1892. 



