MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 141 



But as circumstances are now, the total darkness, the temperature, the degree of dryness or 

 moisture, and other physical conditions remaining the same, the cave fauna is almost completely 

 isolated from that of the upper world ; indeed, far more so than the deep sea fauna of the ocean 

 or of lakes, or the faunas of deserts or of the polar regions, or the alpine inhabitants of lofty mount- 

 ain summits. We thus realize that isolation may be a not unimportant factor in securing perma- 

 nence of type, after the typical characters have once been established through adaptation and 

 heredity. 



After reflecting upon the influence of isolation upon cave animals as securing permanence of 

 varietal, specific, and generic characters, one is led to realize as never before the importance of 

 geographical isolation iu general as a factor in preventing variation after the organisms have once 

 become adapted to their peculiar environment, whether dependent on temperature, soil, humidity, 

 or dryness, the absence of light, or any other appreciable characteristic in their surroundings. 

 We know also that the existing desert, deep-sea, and polar faunas are the product of Quaternary 

 times : that: they were nearly contemporaneous in origin with the cave faunae, though the deep sea 

 fauna may date from the Cretaceus period. Finally, I may quote from Darwin's "Origin of Species" 

 the following extract, which applies (though he did not make it applicable to any special case) 

 with peculiar force to cave fauna : " If, however, an isolated area be very small, either from being 

 suiTOuuded by barriers, or from having very peculiar physical conditions, the total number of the 

 inhabitants will be small, and this will retard the production of new species through natural se- 

 lection, by decreasing the chances of the appearance of favorable individual differences" Fifth 

 edition, New York, p. 105). 



HEREDITY. 



The action of this all-powerful factor in evolution is as constant in the underground world, and 

 as difficult to comprehend iu considering cave life, as that of the upper regions. It begins to act, 

 of course, with the earliest generations, and continues to act with, so to speak, increasing force 

 and precision as time goes on and the characteristics induced by a life in total darkness become 

 more and more fixed. 



It is evident that heredity has acted longest in those insects, such as the species of Anoph- 

 thalmus and Adelops, whose larvae are lacking in all traces of eyes and optic nerves and lobes. 

 Heredity has here acted with uuabated force throughout every stage of the metamorphosis; and, 

 it will be a matter of great interest to ascertain whether any traces of the eyes may be met with 

 in the embryo of these forms. 



On the other hand, in those Arthropods in which the brain and optic nerves have persisted, 

 with rudiments of the eyes (e.g., Orconectes), where the eyes are larger in the young, it would seem 

 as if heredity had been acting through a shorter period, and consequently, so to speak, with less 

 momentum. 



In the case of Machaerites, in which the females only of certain species are said to be blind, 

 while the males have well-developed eyes, we have an apparent exception to the continuous action 

 of heredity : an exception paralleled, however, by animals living in the upper world, such as Termes, 

 whose workers and soldiers are eyeless, though the males and females are e.ved. As will be seen 

 in the section on apparent exceptions to the action of disuse from the effects of darkness, we need 

 further knowledge as to the exact distribution in caves of the species of Machaerites. They perhaps 

 are twilight species rather than inhabitants of totally dark localities in caves, and those living in 

 twilight may intercross with those inhabiting the darker regions, and such a case as this, remark- 

 able as it would appear, does not affect the general rule, that animals living in total darkness 

 and never living in twilight, nor intercrossing with twilight forms, are eyeless, or at least blind. 



Nor does the case of Hadenoecus, the cave cricket, with well-developed eyes and brains, affect 

 the argument; for this is' essentially a twilight form, though migrating to regions of total darkness 

 and abounding there. The same may be said of the cave species of Ceuthophilus. A parallel case 

 may be that of Chologaster as compared with Amblyopsis, the former living out of caves in 

 ditches as well as in wells and caves. 



Judging by the following statement, so eminent a naturalist as Professor Semper denies that 

 heredity acts in the case of the mole. He says : 



