66 ANIMAL COLORATION. 



the ocean abysses, no less than in tlie caves, but the temperature 

 is much lower in the ocean. Corresponding to these physical 

 environmental differences, we might expect to meet with 

 differences in the faunas of the two localities, coupled, how- 

 ever, with certain resemblances due to a certain similarity 

 between the localities. 



And this is precisely what investigations into the fauna of 

 caves show to be the case. 



The darkness of the caves has produced an effect upon the 

 cave animals, just as the darkness of the deep waters of the 

 ocean has upon the abyssal fauna. The structure of the eye 

 and- brain in many cave animals has been worked out by Dr. 

 Packard as well as by others; it would be out of place here to 

 do more than give the general results of these inquiries, which 

 show that there are various stages of degeneration to be met 

 with, culminating in a total disappearance of the organs of 

 vision. In the same way there is an absence of chlorophyll- 

 bearing plants in both places ; the common Hydra and the fresh- 

 water sponge flourish in some caves, but they are entirely' 

 without chlorophyll. As to plants. Dr. Packard finds that not 

 only are Alg« and the higher green plants absent from the 

 caves, to which their spores and seeds must nevertheless have 

 easy access, but even Fungi, independent though they are of 

 light, do not flourish there; with a few exceptions, including 

 Oozonium auricomum and a colourless Agaric, there were no 

 fungi in the caves explored by Dr. Packard. 



The animals of the cave fauna belong to all the orders of terres- 

 trial and fresh-water animals; there is no restriction, anymore 

 than there is in the case of the abyssal fauna, of particular 

 groups to the caves. 



A very imj)ortant difference between cave animals and 

 abyssal marine animals is in their colour, and this is, of course,. 



