54 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



which sufficed to level gradually, bat completely, the uneven 

 mud at the bottom of the aquarium. Now it is true that we 

 know that many currents occur in the ocean which reach to 

 the bottom, and which, as they take their rise at the surface in 

 remote localities, seem specially adapted to convey food from 

 above to the deep-sea creatures. But Mb bins himself points 

 out that one assumption is still unproved by this namely, that 

 the organic portions of the plants and animals that fall from 

 the surface must remain undecomposed in their journey if they 

 are actually to serve as food for the animals there. Whether 

 this is the case we do not know, and this really ingenious theory 

 remains for the present unproved, and may perhaps be ere long 

 replaced by another not less plausible. 



We will now enquire more closely into a few striking 

 phenomena connected with the monophagous habits of certain 

 animals, and endeavour to demonstrate, by the discussion of a 

 few examples, the extremely diverse conditions which appear to 

 be produced by the adaptation of various animals to one single 

 kind of food. It is clear that an animal to whose existence one 

 particular sort of food is indispensable must be the slave of 

 that plant or animal which alone can supply it ; such a mono 

 phagous creature must consequently, in many cases, be adapted 

 to the same mode of life as the organism on which it lives. 

 Many birds live, as is well known, exclusively on hard seeds. 

 ISTow, as the beak of a bird is but rarely adapted to crush such 

 seeds or grains, the grain-eating birds must possess another 

 organ with which to reduce them. This organ is what is 

 known as the gizzard. This has on its ini)er side a veiy thick, 

 hard, brown skin, which is admirably suited to triturate the 

 hard grains by the aid of the grains of sand and small pebbles 

 which are swallowed at the same time, and to protect the softer 

 portions of the stomach against the ill effects of the sand and 

 stones. Thus we here find a peculiarity which enables its pos- 

 sessor to avail itself of a particular supply of food, which the birds 

 of prey with their soft stomachs are unable to take advantage of. 

 A still more striking illustration of the fact that such organs', 

 calculated for a single kind of food, sometimes appear under 

 very unexpected aspects, such as by all school theories would be 



