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 iu 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 plainly 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 iu 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 and-in breed¬ 
ing within tne narrow fixed limits of the cave, and are not these collectively verce causce; 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 animhls 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 taxonomical 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, Oaecidotsea, Pseudotremia, Scoterpes, Anophthahnus, 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 iuto 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 reunite 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 genera are on the whole quite as characteristic 
and distinct as hundreds of genera and subgenera not called iu 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 comiug to an agreement as to the exact limits of some of the species and genera 
peculiar to caves. 
ON THE EMPLOYMENT OP 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 genera 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 easdy 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 founded 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 
