332 PHYSIOLOGICAL DEVELOPMENT. 



gout and rheumatism have their habitual places of deposit. 

 Tuberculous matter, though it may be present in various 

 organs, gravitates towards some much more than towards 

 others. Certain products of disease are habitually got rid of 

 by the skin, instead of collecting internally. Mostly, these 

 have special parts of the skin which they affect rather than 

 the rest; and there are those which, by breaking out sym- 

 metrically on the two sides of the body, show how definitely 

 the places of their excretion are determined by certain 

 favouring conditions, which corresponding parts may be pre- 

 sumed to furnish in equal degrees. Further, it is to be 

 observed of these morbid substances circulating in the blood, 

 that having once commenced segregating at particular places, 

 they tend to continue segregating at those places. Assum- 

 ing, then, as we may fairly do, that this localization of 

 excretion, which we see 'continually commencing afresh with 

 morbid matters, has always gone on with the matters pro- 

 duced by the waste of the tissues, let us take a further 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, survival of the fittest 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 still more 

 decisively by survival of the fittest. 



Such being the 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 



