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PHYSIOLOGICAL DEVELOPMENT. 



step, and ask how localizations become fixed. Other things 

 equal, that which from its physical conditions is a place of 

 least resistance to the exit of an effete product, will tend to 

 become established as the place of excretion ; since the rapid 

 exit of an effete product will profit the organism. Other 

 things equal, a place at which the excreted matter produces 

 least detrimental effect will become the established place. If 

 at any point the excreted matter produces a beneficial effect, 

 then, other things equal, natural selection will determine it 

 to this point. And if facility of escape anywhere goes along 

 with utilization of the escaping substance, then, other things 

 equal, the excretion will be there localized by survival of the 

 fittest. 



Such being tlic conditions of the problem, let us ask what 

 will happen with the lining membrane of the alimentary 

 canal. This, physiologically considered, is an external sur- 

 face ; and matters thrown off from it make their way out of 

 the body. It is also a surface along which is moving the food 

 to be digested. Now, among the various waste products 

 continually escaping from the living' tissues, some of the 

 more complex ones, not very stable in composition, are likely, 

 if added to the food, to set up changes in it. Such changes 

 may either aid or hinder the preparation of the food for 

 absorption. If an eflfete matter, making its exit through the 

 wall of the intestine, hinders the digestive process, the 

 enfeeblement and disappearance of individuals in which this 

 happens, will prevent the intestine from becoming the esta- 

 blished place for its exit. While if it aids the digestive 

 process, the intestine will, for converse reasons, become more 

 and more the place to which its exit is limited. Equally 

 manifest is it that if there is one part of this alimentary canul 

 at which, more than at any other part, the favourable effect 

 results, this will become the place of excretion. If 



from this general statement we pass to the special case 

 before us, we find our data to be these : — The substance to be 

 excreted, biliverdine, a waste product of the organic actions, 



