NOTES. 415 



ing, so that the former occurred only in birds that feed on grain, 

 and the latter in those that feed on animal food. This, however, 

 would be an error, since there is a considerable number of flesh-eating 

 birds of which the stomach (the muscular stomach, as it is called) is 

 constructed exactly like that of a pigeon or a hen. Pudioeps minor 

 (the little Grebe) lives on fishes, worms, and soft-bodied aquatic 

 creatures. Corvus Cm-nix (the hooded crow) and C. Curax eat insects, 

 birds, and small quadrupeds. The lapwing lives on soft aquatic animals, 

 and the kingfisher on fish. In all these the muscular stomach has quite 

 as thick a muscular layer as in the pigeon, and the internal coat is a 

 hard, brown pseudo-cuticle, such as is always presentinthegraminivorous 

 birds. Even among true birds of prey some have the pseudo-cuticle, at 

 any rate for a time — as the recently fledged kestrel — though not, it is 

 true very strongly developed. In these birds a meat diet does not seem 

 to effect so rapid a change in the stomach— from a graminivorous to a 

 carnivorous type — as in the gull and the pigeon, nor, indeed, to affect 

 them in any way. 



Note 15, page 69. In the text I have not mentioned a number of 

 effects of food which in part are not suited for discussion in a popular 

 lecture, but in part too have no bearing on the question we must steadily 

 keep before us, i.e. how far the maintenance of the species or the origin 

 of new forms may be induced by them. Thus, for instance, Toit's ex- 

 periments on the assimilation of fresh-water mussels seem to have 

 established beyond a doubt that the greater part of the ashy consti- 

 tuents of their bodies, which they deposit almost exclusively in 

 their shel'.s, are derived from the water which, according to him, is 

 taken up by the kidneys. But then the question as to how the water 

 penetrates the body of the mussel is still under controversy. Some 

 authors entirely set aside the old view that the kidneys, wholly or in 

 part, have the function of absorbing water : according to them it takes 

 place through pores in the skin ; but this, notwithstanding all that 

 has been said on the subject, is not absolutely beyond dispute. In 

 fact, so long as the morphological bases of physiological speculations 

 are as little assured as in this case, any discussion must be regarded as 

 premature. 



The second point, to be here only lightly touched on, regards the 

 influence of food on the sexual functions and in the external distinc- 

 tions of sex which partly depend on it. It is known that certain foods 

 or stimulants at the approach of sexual maturity have a stimulating 

 effect on the secretion of the semen. Too small a supply of food is as 

 injurious to the germinal glands as too large a one ; for this predomi- 

 nantly important side of animal life, also, there is an optimum of nutri- 

 tion, and any excess towards the maximum, or deflciency towards the 

 minimum, must exert a proportionally injurious influence. Unfortu- 

 nately we know of hardly any serviceable experiments on this subject, 



