VI FORM AND FUNCTION 65 
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 Amceba (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 are 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. Ihave seen stones of porten- 
tous size which had been taken from the gizzard of an 
Emceu. 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 
F 
