vi FORM AND FUNCTION 6$ 



process quite different from Osmosis. The living 

 membrane has a power of selection : it is like a sieve 

 which can let big molecules pass, while it can reject 

 smaller ones. Each cell seems, like an Amoeba (of 

 which more presently), to have the power of choosing 

 out and swallowing what it wants. In the same way 

 plants select their food from the ground. Much of 

 the everyday work of nature is too subtle for science 

 to explain. 



When the food has penetrated into the blood-vessels 

 it is no longer a foreign substance, but having been 

 thoroughly assimilated has become part of the bird 

 itself. As a rule, however, the process of assimilation 

 is not completed in the proventriculus. The food 

 passes on to the second compartment of the stomach, 

 the walls of which, in seed-eating birds especially, are 

 very thick and strong, being formed of muscular fibres 

 which radiate out from two tendons running down the 

 centre of each side. No less powerful mill would be 

 equal to the grinding of acorns, and even this would 

 be insufficient did not the bird swallow stones which, 

 like molar teeth, break up the food as the muscles 

 contract and relax. So necessary are such molars, 

 that where no stones arc to be had birds have been 

 known to swallow hard stonelike seeds, for instance 

 those of the wild prairie rose (Rosa Blanda) which 

 fulfil the same purpose. I have seen stones of porten- 

 tous size which had been taken from the gizzard of an 

 Emeu. In birds which live on flesh the walls of the 

 stomach are very weak, so that it does not deserve the 

 name of a gizzard and, moreover, no stones are 

 swallowed, nothing of the nature of teeth being 



