214 THE STRUCTURE OF MAX 



of time for their accomplishment, so that, as a rule, they are 

 removed from direct perception by means of the senses, and can 

 only be inferred from the evidence of Phylogeny, Comparative 

 Anatomy, and Ontogeny. 



This applies not only to Man, but to the whole animal 

 kingdom, which yields us a long series of examples of degenera- 

 tion. Here also we find evidence of the great importance of the 

 external conditions of life to which the organism responds. One 

 of the most striking proofs of this is afforded by the degenerate 

 condition, or even entire absence, of eyes in animals living in 

 the depth of the ocean or in caves. Such animals also illustrate 

 how the loss of one organ is compensated for by the increased 

 development of other organs. From the same point of view are 

 to be considered the limbless Amphibia, and the Slow-worms, 

 and another group of Eeptiles of essentially similar adaptive 

 organisation, the Amphisbsenidee, and finally the more familial- 

 Earthworm itself. 



Whereas, among the above-mentioned cases, it is the organ 

 of sight which atrophies ; in other animals, the olfactory organ 

 disappears, and I may especially refer to those Fishes known, 

 from the characters of their jaws and teeth, as the Plectognathi 

 Gymnodontes. Here, 1 in adaptation to a diet of Crustacea and 

 Molluscs which are very difficult to crush, the musculature of the 

 jaws develops to an extraordinary degree, displacing the olfactory 

 apparatus to such an extent that the olfactory nerve is reduced 

 to a minute thread, which branches either within a mere tegu- 

 mental olfactory process or simply under the surface integument 

 of the olfactory region. 



Until quite recently, the question wherein lay the cause of 

 the degeneration of an organ was thought to be satisfactorily 

 answered as follows : the organ is not used, and the degenerating 

 effect of disuse, passed on from one generation to another, gains 

 in intensity, until it leads to the total removal of the organ in 

 question. This answer presupposes what is often stated, but has 

 never been proved, viz. the inheritance of acquired characteristics. 2 



1 Cf. "Wiedersheim, "Das Geruchsorgan der Tetrodonten." Kolliker Gratula- 

 tionsschrift, 1887. 



2 [This statement requires qualification. It is true that we have no very satisfac- 

 tory concrete instance of a chance structural modification of an individual having been 

 transmitted by inheritance to its own immediate offspring. But, on the other hand, 

 as Herbert Spencer has argued with great force, there seems no way of explaining 

 the phenomena of highly organised life, except on the supposition of some transmis- 

 sion of characters acquired in adaptation to the environment. ] 



