44 
aqueous and vitreous bodies and the lens are not essential for the per- 
formance of the function of the eye, these structures cease to develop 
early. The processes of degeneration follow the same rate. Degeneration 
is brought about by the falling apart of the elements as the result of the 
introduction of connective tissue cells that act as wedges. Abnormal de- 
generation sometimes becomes manifest through the cessation of the re- 
duction of parts (p. 269) that normally decrease in size, so that these parts 
in the degenerate organ are unusually large. 
Kohl’s theoretical explanation here given somewhat at length is based 
on the study of an extensive series of degenerate eyes. He has not been 
able to test the theory in a series of animals living actually in the condi- 
tion he supposes for them, and has permitted his erroneous interpretation 
of the highly degenerate eye of Troglichthys to lead him to the theory 
of the arresting of the eye in ever earlier stages of ontogeny. The eye of 
Troglichthys is in an entirely different condition from that supposed by 
him. The mere checking of the normal morphogenic development has 
done absolutely nothing to bring about this condition, and it could not 
have been produced by the checking of development in ever earlier and 
earlier stages of ontogeny, for there is no stage in normal ontogeny re- 
sembling in the remotest degree the eye of Troglichthys. The process of 
degeneration as seen in the Amblyopsidae is in the first instance one of 
growing smaller and simpler (not more primitive) in the light, not a cut- 
ting off of late stages in the development in the dark. The simplified con- 
dition, it is true, appears earlier and earlier in ontogeny till it appears 
along the entire line of development, even in the earliest stages. The ten- 
dency for characters, added or modified at the end of ontogeny, to appear 
earlier and earlier in the ontogeny is well known and there is no inherent 
reason why an organ disappearing in the adult should not eventually dis- 
appear entirely from ontogeny. The fact that organs which have disap- 
peared in the adult have in many instances not also disappeared in the 
ontogeny, and remain as so-called rudimentary organs, has received an ex- 
planation from Sedgwick. In his re-examination of the biogenetic law he 
came to the conclusion that “the only functionless ancestral structures, 
which are present in development, are those which at some time or an- 
other have been of use to the organism during its development after they 
have ceased to be so in the adult.” 
The length of time in such cases since the disuse of such an organ in 
the young is much shorter than that since its disuse in the adult. 
