THE CAUSES OF INDIVIDUAL AND PHYLETIC 

 DEGENERATION. 



It may now be profitable to take up the causes leading to the small degree of 

 degeneration found in Chologaster, the degeneration of the eye in Amblyopsis, 

 Typhlichthys, and Troglichthys to a mere vestige, together with the total disap- 

 pearance of some of the accessory structures of the eye, as the muscles, in some of 

 the species. In the outset of this consideration we must guard against the almost 

 universal supposition that animals depending on their eyes for food are or have been 

 colonizing caves, or that the blind forms are the results of catastrophes that have 

 happened to eyed forms depending on their eyesight for their existence. This idea, 

 so prevalent, vitiates nearly everything that has been written on the degeneration 

 of the eyes of cave animals. 



The degeneration of organs ontogenetically and phylogenetically has received 

 a variety of explanations. 



(1) The organ diminishes with disuse (ontogenetic degeneration, Lamarck, 

 Roux, Packard) and the effect of this disuse appears to some extent in the next gen- 

 eration (phylogenetic degeneration, Lamarck, Roux, Packard). 



(2) Through a condition of panmixia the general average maintained by selection 

 is reduced to the birth mean in one generation (ontogenetic, Romanes, Lankester, 

 Lloyd Morgan, Weismann) to the greatest possible degeneration in succeeding 

 generations (phylogenetic, Weismann), or but little below the birth average of^the 

 first generation (Weismann's later view, Romanes, Morgan, Lankester). 



(3) Through natural selection (reversed) (the struggle of persons) the organ may 

 be caused to degenerate either (a) by the migration of persons with highly developed 

 eyes from the colony living in the dark (Lankester), or (b) through economy of 

 weight and nutriment or liability to injury (phylogenetic purely, Darwin, Romanes). 



(4) Through the struggle of parts (a) for room an unused organ in the individual 

 may be crowded (ontogenetic, Roux), (b) for food, this may lead to the development 

 of the used organ as against the disused through a compensation of growth (Goethe, 

 St. Hilair, Roux) ; this ontogenetic result becomes phylogenetic through transmis- 

 sion of the acquired character (Roux), or is in its very nature phyloblastic (Kohl). 



(5) Through the struggle between soma and germ to produce the maximum 

 efficiency of the former with the minimum expenditure of the latter (ontogenetic 

 and phylogenetic, Lendenfeld). 



(6) Through germinal selection, the struggle of the representatives of organs 

 in the germ (ontogenetic and phylogenetic, Weismann). 



(7) To these special considerations should be added the recently suggested gen- 

 eral process of mutation. 



The idea of ontogenetic degeneration is intimately bound up with the idea of 

 phylogenetic degeneration. Logically we ought to consider first the causes of indi- 

 vidual degeneration and then the processes or causes that led to the transmission 



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