82 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



and that this occurs to each individual separately within the 

 period of its separate life. These, however, as every zoologist 

 knows, are not the only cases. Most of the blind Paiusitical 

 Crustacea now extant have larvae with well-developed eyes ; the 

 young form of many worms, Parasites on Mollusca, &c. 

 (Trematoda), can see, though the adult individuals are blind. 

 In the greater number of these cases as, for instance, in all 

 internal parasites we must refer the loss of sight to the same 

 above-mentioned cause, namely, disuse of the organ. 



But though we are thus fully justified in saying that dark- 

 ness so complete as not to allow of the eyes being used at all 

 has in most cases exercised an injurious effect on their existence 

 and structure, it would nevertheless be wholly false to assume 

 that the lack of light must necessarily lead to total or partial 

 blindness. We know of a number of facts directly opposed to 

 such a conclusion. Among the numerous cave-insects there 

 are many which have well -developed eyes, and yet inhabit the 

 same spot as blind species. In some caves in the Philippines 

 and the Pelew Islands which I myself explored, I found, in 

 spots where the most absolute and total darkness reigned, only 

 insects with eyes ; Hadencecus, a species of grasshopper which 

 lives in the caves of Kentucky, has well -developed eyes like 

 other animals found there at the same time. 20 Why should 

 not darkness have had the same effect on these animals as 

 on others which have in fact become blind] It might be 

 said in fact it has been said that the cave-animals which 

 can see have migrated into the cave only within a short period, 

 and have not been exposed to the influence of the darkness 

 long enough to suffer ; while the blind or half-blind, having 

 entered the caves at a remote period, have lost the use of their 

 eyes, wholly or partially, in consequence of long desuetude. 

 But this explanation contradicts the fact previously mentioned, 

 that every mole, Pinnotheres, &c., originally had eyes apparently 

 capable of further development, and of perfectly fulfilling their 

 normal function ; and that the influence of darkness is proved 

 to be direct in each individual, and not hereditary. This ex- 

 planation is also quite decisively contradicted by a fact which is 

 little known generally, and even among zoologists is familiar 



