104 FAUNA OF MAYFIELD'S CAVE. 



same manner that Anopthalmus differs from Trechvs and that Orconectes 

 differs from Camharus. Csecidotea is more slender, lacks eyes and pig- 

 ment, and has longer and more slender appendages, but is otherwise 

 much like Asellvs. The genus Caecidotea is probably as good as Orco- 

 nectes or Anopthalmus, and no better. It is generally accepted, however. 

 On the other hand, these genera are just as good as hundreds of others 

 which have met with general acceptance and which are not questioned. 



CAUSES LEADING TO THE MODIFICATIONS OF CAVE ANIMALS. 



At present I do not care to enter into any extended discussion of 

 the causes of the modifications of cave animals. The more slender 

 appendages, the more slender bodies, and the increase in number 

 or development of the sense-papillae and sensory setae in cave ani- 

 mals may be due to individual adaptations or they may be due to nat- 

 ural selection tending to eliminate all individuals which did not possess 

 variations of assistance to the animal under its unusual conditions. 



It seems to me no satisfactory explanation for the loss of pigment 

 and degeneration of the eyes in cave animals has been offered, unless 

 we grant the influence of the environment upon the individual, together 

 with the hereditary transmission of these acquired characters. The 

 validity of the transmission of the effects of disuse is the contention of 

 Spencer (1893), Packard (1888, 1894), and Eigenmann (1899, 1903); 

 but as Eigenmann (1903, 197) admits: 



There has always been and is yet a serious objection to the latter conclusion [i. e. , 

 the hereditary transmission of the effects of disuse upon the eye], because the method 

 of the transmission of functional adaptations to the organization of the egg so as to limit 

 or extend its powers is not known. 



Until experiments have shown that an outside form kept under cave 

 conditions may be modified by its life there and this modification trans- 

 mitted to its offspring, the explanation of the origin of blind and color- 

 less cave forms through the heredity of the effects of disuse and of the 

 influence of lack of light can not be very satisfactory. 



Adaptive structures are explained by Osborn as follows (1897, 584) : 



All the individuals of a race are similarly modified over such long periods of time 

 that very gradually congenital or phylogenetic variations which happen to coincide 

 with the ontogenetic adaptive variations are selected. 



Further, the same authority says (1897, 585) : 



The law of determinate variation is observed to operate with equal force in cer- 

 tain structures, such as the teeth, which are not improved by individual use or exercise 

 as in structures which are so improved. 



He is thus led to a belief in "fundamental predispositions to vary 

 in certain directions." 



