214 THE STRUCTURE OF MAN 
of time for their accomphshment, 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 lmbless Amphibia, and the Slow-worms, 
and another group of Reptiles of essentially similar adaptive 
organisation, the Amphisbeenide, and finally the more familar 
Earthworm itself. : 
Whereas, among the above-mentioned cases, it 1s 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,’ in adaptation to a diet of Crustacea and 
Molluses 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.” 
1 Cf. Wiedersheim, ‘‘ Das Geruchsorgan der Tetrodonten.” Kolliker Gratula- 
tionsschrift, 1887. 
? [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. ] 
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