inconvenienced by its movements. The causation of tinctumutation is now definitely known. The theory that light acts directly on the chromatophoric cells has been proved to be incorrect. Even the theory that light occasions pigmentation is no longer tenable. I have, time and again, reared tadpoles from the eggs in total darkness, yet they differ in no respect from those reared in full daylight. The chromatophores were as abundant and responded to irritation as promptly in the one as in the other. The distinguished Paul Bert declared that the young of the axolotl could not form pigment when reared in a yellow light. Professor Semper, on the contrary, declares Bert's axolotls to be albinos, and states that albinism is by no means infrequent in the axolotl; also that Professor Kölliker, of Würtzburg, reared a family of white axolotls in a laboratory where there was an abundance of light, and that he (Semper) never succeeded in rearing an albino, though there was less light in his laboratory than in that of Kölliker, and his axolotls came from the same stock. Bert made the mistake of confounding albinism with the phenomenon of etiolation in plants; in fact, he gives the name "etiolation" to the albinism noticed in his axolotls.[95] [94] Semper, _Animal Life_, p. 93. [95] _Ibid._, p. 88 _et seq._ There is a marked difference between the functions of the chlorophyll bodies found in plants and the chromatophores found in animals. The former play one of the most important rôles in the drama of plant life, inasmuch as they subserve a vital function, while the latter act a minor part, because they serve only as an instrument or means of protection. Light is of great importance in its influence on chlorophyll, which is a microscopic, elementary body on which the vital strength of the plant depends, while it is not at all necessary to the chromatophores,--cell bodies secreting pigmentary matter for the purpose of protection. Of course, when animals are subjected to darkness for very long periods of time, the chromatophores are modified, and, sometimes, are wholly obliterated. They follow a well-known natural law, which declares that, when a function of an organ is no longer of any use to an animal, both organ and function become rudimentary, and finally disappear. Many animals live for generations in total darkness before losing their pigment. I, myself, have seen black beetles in Mammoth Cave, Kentucky, in the neighborhood of Gorin's Dome, which is far within the depths of the cave. As beetles rarely range over a hundred yards from their place of birth, these insects must have been born in the cave and reared in the dark. When speaking of light, if not otherwise specified, I mean diffused daylight which carries no heat rays. I believe that heat is a prominent factor in the production of color; the discussion of this point, however, does not properly belong to the subject under consideration.