90 IHE INFLUENCE OF INANIMATE SURROUNDINGS. 



the pigment of the skin of the tadpoles was normally developed 

 in every kind of light, as also in the dark. 



Thus experiment here confronts experiment. It is not 

 difficult to find an hypothesis to account for this. In none of 

 the experiments hitherto conducted, not even those of Bert, 

 were the heat-rays or the chemical rays excluded from the light 

 falling on the young animals. It may have happened that in 

 the darkness the little larvse were not supplied with supple- 

 mentary or even requisite nourishment in short, it would seem 

 that the absence of pigment observed by Bert in the young 

 Axolotl did not arise from the absence of light, but from the 

 effects of some other cause as yet not ascertained, as insufficient 

 or unsuitable food, the sinking or raising of the temperature, &c. : 

 or it was perhaps a case of true albinism, and thus a form of 

 disease. All "who have bred the Mexican Axolotl are well 

 aware that sometimes a white variety not a true albino 

 suddenly occurs ; but the cause of this variation is at present 

 unknown. Thus Professor Kolliker of Wiirzburg reared a 

 whole family of these white Axolotls, which, with their blood 

 red gills, were very beautiful objects ; while in my own labora- 

 tory, where there is a much greater absence of light than in 

 Kolliker's, I have as yet entirely failed in breeding even one 

 white Axolotl, although during the last six years I have bred 

 hundreds of individuals under the most various conditions of 

 life. I am wholly unable to assign any plausible explanation 

 for this difference, and it is the more striking because the six 

 old specimens, from which I have now had at least six or seven 

 broods, came originally from the same brood as those from 

 which Kolliker has obtained so many white individuals. 

 Finally I can but repeat my conviction, founded on these experi- 

 ments, first, that we have as yet no suspicion even of the causes 

 which sometimes determine the absence of the epidermal pig- 

 ment in the Amphibia and other animals (as rats and mice, in 

 which these unknown causes even become hereditary) ; and 

 secondly, that this absence of colour is certainly not to be 

 ascribed to the absence of light, since we know that animal 

 pigment, like vegetable pigment, can be developed in total 



