MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 121 



has been united to Trechus, since there is a series of forms with more or less rudimentary eyes 

 connected with the eyed species of Trechus. Bedel also tells us that in all the species of Trechus 

 there is a natural tendency to penetrate into grottoes, even when ordinarily they live in the open 

 air buried in the earth under stones. 



It seems reasonable to conclude that the cave species, which are without optic ganglia, optic 

 nerves, and any traces of eyes had originally, by adaptation to total darkness, become isolated, 

 and that their characteristics after being fixed by heredity have been transmitted for generations, 

 becoming as unchanging in their way as the physical conditions of darkness and uniform temper- 

 ature surrounding them. Those living in the open air in the soil under stones, or at or just within 

 the entrance to caves, vary most as regards the eyes, as we have found to be the case with the 

 other forms previously mentioned. 



This intimate dependence on the physical conditions of life is so plaiuly shown in these ani- 

 mals, that we can well understand how potent have been the factors (i. e., change from light to 

 total darkness and an even cave temperature) which have operated on out-of-door forms to induce 

 variation, (riven great changes in the physical surroundings, inducing loss of eyes from disuse, 

 the abolition in some cases of the optic ganglia and optic nerves, the elongation of the appendages, 

 isolation from out of door allies, and the transmission by heredity owing to close in aud-iu breed- 

 ing within the narrow fixed limits of the cave, and are not these collectively verm causw ; do 

 they not fully account for the original variations and their fixation; in short, can we not clearly 

 understand the mode of origin of cave species and genera? What room is there in a case like 

 this or in that of parasitic animals for the operation of natural selection? The latter principle 

 only plays, it has seemed to us, a very subordinate and final part in the set of causes inducing the 

 origin of these forms. 



It is to be observed that from a taxonoraical point of view the classification of nearly all the 

 typical cave forms is in a state of uncertainty. What we are disposed to regard as distinct genera, 

 such as Orconectes, Caecidotaea, Pseudotremia, Scoterpes, Anophthalmus, Bathyscia, and others, 

 have in some cases been referred to older-established well-known genera whose species live out 

 of doors under ordinary circumstances. To enter into particulars would lead us into details 

 already given under the head of these groups in the systematic portion of this work. But it is 

 still an open question whether the above-mentioned genera are "good" genera. The tendency 

 among systematists is to reuuite the eyeless forms to the eyed genera if specimens with rudiment- 

 ary eyes are found to connect them. We would draw attention to this point, for it is at present 

 the opinion of a few naturalists that the eyeless geuera are on the whole quite as characteristic 

 and distinct as huudreds of genera and subgenera not called in question. The point now insisted 

 on is the instability of the genera and certain of the species in question, and the difficulty felt by 

 naturalists in coming to an agreement as to the exact limits of some of the species and genera 

 peculiar to caves. 



ON THE EMPLOYMENT OF CERTAIN GENERIC NAMES FOR BLIND ANIMALS. 



The greater the increase of our knowledge of the species and varieties of eyeless genera as 

 compared with their eyed allies, the more marked becomes the tendency of some writers to unite 

 them in the same generic group. There is also the same general tendency to unite species and 

 slightly separated geuera the larger become our collections and the more widely extended our 

 knowledge of species and the links connecting them. This tendency may be carried too far, and 

 while we recognize the highly probable fact that certain genera may have been evolved from 

 other, perhaps older and better known, generic forms, as a matter of convenience it is better, we 

 think, to have names for such collections of species. They may be regarded as genera, or, if the 

 stem-genus, so to speak, is a bulky, unwieldy collection of species, we may within due limits 

 regard the more aberrant and easily recognized species as forming one or more subgenera. In 

 the case of blind or eyeless "genera" it is certainly a great convenience to use special names for 

 such groups of species. They are fouuded on characters which are certainly of more fundamental 

 importance than hundreds and even perhaps thousands of genera which pass current at the present 

 day, and are founded on characters of trivial importance. For example, let us take the old and 

 usually accepted genus Anophthalmus. It was separated from Trechus on account of being eyeless 



