39o THE POPULAR SCIENCE MONTHLY. 



animals, the eyes are wanting froni causes of the same nature as 

 have induced their absence in true cave animals. No animal or 

 series of generations of animals, wholly or in part, lose the organs 

 of vision unless there is a physical appreciable cause for it. "While 

 we may never be able to satisfactorily explain the loss of eyes in 

 certain deep-sea animals from our inability to personally pene- 

 trate to the abysses of the sea, we can explore caves at all times of 

 day and night, of winter and summer ; we can study the egg-lay- 

 ing habits of the animals, and their embryonic development ; we 

 can readily understand how the caves were colonized from the 

 animals living in their vicinity ; we can nicely estimate the na- 

 ture of their food, and its source and amount, as compared with 

 that accessible to out-of-door animals ; we can estimate with some 

 approach to exactitude the length of time which has elapsed since 

 the caves were abandoned by the subterranean streams which 

 formed them and became fitted for the abode of animal life. The 

 caves in southern Europe have been explored by more numerous 

 observers than those of this country, and the European cave 

 fauna is richer than the American, but the conditions of Euro- 

 pean cave life and the effects of absence of light and the geological 

 age of the cave fauna are like those of American caves. More- 

 over, the cave life of New Zealand and the forms there living in 

 subterranean passages and in wells show that animal life in that 

 region of the earth has been affected in the same manner. The 

 facts seem to point to the origin of the cave forms from the species 

 now constituting a portion of the present Quaternary fauna; 

 hence they are of very recent origin. 



The result of cave exploration shows that no plants, even the 

 lowest fungi, with the exception of Oozonium auricomum Link, 

 and perhaps one or two other kinds of fungi common to Europe 

 and America in and out of caves, can so adapt themselves as to 

 live and propagate their species in the total darkness of caverns. 

 They are far more dependent on the influence of light than 

 animals. 



"We will now briefly rehearse the facts relating to the changes 

 in structure and color undergone by animals adapted to a life in 

 total darkness in caves, premising that, so far as we know, the 

 Protozoa detected in subterranean waters do not essentially differ 

 from those living in the light. It appears from the following 

 facts that eyeless animals change their color as well as those hav- 

 ing eyes : 



1. A sponge {Spongilla stygia) found by Dr. Joseph in the 

 waters of Carniolan grottoes, instead of being green, is pellucid 

 and bleached. 



2. The Hydra (H. pellucida), also found by Dr. Joseph in the 

 subterranean lakes of Carniola, was, as its name indicates, neither 



