﻿236 BLIND VERTEBRATES AND THEIR EYES. 



of this. Practically it is impossible to do so, because many of the explanations are 

 general. Only number (4) of the above may be taken in the ontogenetic sense purely, 

 though it was certainly also meant to explain phylogenetic degeneration. In many 

 of the explanations of particular cases of degeneration more than one of the above 

 principles are invoked, though only one was meant to be used. In most cases, how- 

 ever, the discussions of degeneration have been in general terms, without direct 

 bearing on any specific instance of degeneration in all its details. It must be evident 

 that such discussions can only by accident lead to right results. 



By the Lamarckian ontogenetic degeneration is considered the result of lack of 

 use and consequent diminished blood supply. The results of the diminution caused 

 by the lack of use during one generation are transmitted in some degree to the next 

 generation, which thus starts at a lower level. A continuation of the same con- 

 ditions leads finally to the great reduction and ultimate disappearance of an organ. 



No one, so far as the author knows, has attempted, or, perhaps better, suc- 

 ceeded, in accounting with this factor in detail for the degeneration of the eye. 

 Packard's explanations are evidently a mixture of Lamarckism and Darwinism. 



Packard says, "When a number, few or many, of normal seeing animals enter 

 a totally dark cave or stream, some may become blind sooner than others, some hav- 

 ing the eyes slightly modified by disuse, while others" may have in addition physi- 

 cal or functional defects, especially in the optic nerves and ganglia. " The result of 

 the union of such individuals and of adaptation to their stygian life would be broods 

 of young, some with vision unimpaired, others with a tendency to blindness, while 

 in others there would be noticed the first steps in degeneration of nervous power 

 and nervous tissue." Packard evidently had invertebrates in mind. He clearly 

 admits the cessation of selection or panmixia which is implied by his supposition 

 that those born with defects may breed with the others. He supposes that the 

 blind fauna may have arisen in but few or several generations, a supposition 

 that may be applicable to invertebrates, but certainly may not be applied to the 

 vertebrates. At first those becoming so modified that they can do without the 

 use of their eyes would greatly preponderate over those " congenitally blind." 

 "So all the while, the process of adaptation going on, theantennce and other tactile 

 organs increasing in length and in the delicacy of structures, while the eyes were 

 meanwhile diminishing in strength of vision and their nervous force giving out, 

 after a few generations, perhaps only two or three, the number of congenitally 

 blind would increase, and eventually they would, in their turn, preponderate in 

 numbers." Packard seems here to admit the principle of degeneration as the 

 result of compensation of growth, the nervous force of the eye giving out with the 

 increase of the tactile and olfactory organs. It is somewhat doubtful in what sense 

 the term " congenitally bhnd " is used, but it probably means born blind as the 

 result of transmitted disuse rather than blind as the result of fortuitous variation. 

 The effects of disuse are thus supposed through their transmission to have given 

 rise to generations of blind animals. The continued degeneration is not discussed. 



In 1873, 1874, and 1890, Romanes, in a series of articles in "Nature" and later 

 in "Darwin and after Darwin," 11, page 291 et scq., maintained that the beginning 

 of degeneration is due to cessation of selection, and continued degeneration to the 

 reversal of selection and final failing of the power of heredity. Selection he supposed 

 to be reversed because the organ no longer of use "is absorbing nutriment, causing 



