March 30, 1900.] 



SCIENCE. 



501 



function of the eye, these structures cease 

 to develop earh^. The processes of degen- 

 eration follow the same rate. Degeneration 

 is brought about by the falling apart of the 

 elements as the result of the introduction 

 oi, connective tissue cells that act as wedges. 

 Abnormal degeneration sometimes becomes 

 manifest through the cessation of the re- 

 duction of parts that normally decrease in 

 size »(p. 269), so that these parts in the 

 degenerate organ are unusually large. 



Kohl's theoretical explanation is based 

 on the study of an extensive series of degen- 

 erate ej'es. He has not been able to test 

 the theory in a series of animals actually 

 living in the condition he supposes for them, 

 and has permitted his erroneous interpre- 

 tation of the highly degenerate eye of Trog- 

 Uchthys to lead him .to the theory of the 

 arresting of the eye in ever-earlier stages 

 of ontogenj'. The eye of Troglichthys is in 

 an entirely' diiferent 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 pro- 

 duced by the checking of development in 

 ever earlier and earlier stages of ontogeny, 

 for there is no stage in normal ontogeny 

 resembling in the remotest degree the eye of 

 the adult Troglichthys. The process of de- 

 generation as seen in the Amblj'opsidpe is in 

 the first instance one of growing smaller 

 and simpler (not more primitive) in the light 

 — not a cutting off of late stages in the de- 

 velopment in the dark. The simplified 

 condition, it is true, appears earlier and 

 earlier in ontogeny till it appears along 

 the entire line of development, even in the 

 earliest stages. This simplified condition 

 never gives any evidence of being more prim- 

 itive ; it is simply less elaborate. The tend- 

 ency 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 disap- 

 pear entirely from ontogeny. The fact that 

 organs which have disappeared in the adult, 

 have in many instances not also disappeared 

 in the ontogeny and remain as so-called rud- 

 imentary organs has received an explana- 

 tion from Sedgwick. In his re-examination 

 of the biogenetic law he came to the conclu- 

 sion that "the onljr functionless ancestral 

 structures, which are present in develop- 

 ment, 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 use in the adult. All 

 organs functionless in the adult, but func- 

 tional in the early ontogeny, develop in the 

 normal way. Organs no longer functional 

 at any time dwindle all along the line of 

 development. In Typhlogobius, where the 

 eye is functional in the young, it develops 

 to full size in the embryo and it is not till 

 late in life that degeneration is noticeable. 

 In Amblyopsis on the other hand, where the 

 ey« has not been functional at any period 

 of ontogeny for many generations where 

 the eye of both the young and adult lost 

 their function on entering the caves, and 

 where degeneration begins at an early period 

 and continues till death, the degenerate 

 condition has reached the early stages of 

 the embryo. It is only during the first few 

 hours that the eye gives promise of becoming 

 anything more than it eventually does be- 

 come. The degree of degeneration of an 

 organ can be measured as readily by the 

 stage in ontogeny when the degeneration 

 becomes noticeable as by the structure in 

 the adult. The greater the degeneration 

 the farther back in the ontogeny the degen- 

 erate condition becomes apparent unless, as 

 stated above, the organ is of use at some 

 time in ontogeny. It is evident that an 

 organ in the process of being perfected by 



