REPAIR IN EVOLUTION 79 



handicap which puts greater general stress on all who 

 experience it, though such stresses fall short of those which 

 cause death. Variations of this order may only be advan- 

 tageous to the whole species as a continuing race. They 

 may destroy, and doubtless have destroyed, individuals 

 without number at an earlier age than the usual life-period 

 of the unvaried type. We may possibly imagine a part of 

 humanity, now responding to stresses which make the 

 heart do more work and fail earlier, displaying such energy 

 during their shorter life as to displace those with a normal 

 cardiac mechanism which survives to the average age of 

 man. It is to be inferred from these considerations that 

 the structure of an organism is not a congeries of minute 

 fortuitous advantageous variations, nor the gradual 

 massing of details in an orthogenetic line, nor the result of 

 large discontinuous variations due to chromosomatic 

 inheritance, but a complex of definite reactions to definite 

 stresses. The true theory of living structure is that its 

 growth is neither casual nor foreseen, but that it is what we 

 may call, in political language, the " opportunism " of the 

 organism as a whole. Every advance is a forced, even a 

 desperate, experiment. Life, like a hypothesis or a dam, 

 is built up by stopping leaks. 



The evolution of the stomach seems to have followed 

 the lines suggested for cardiac development. From the 

 physiological point of view, an intestinal tube which be- 

 comes dilated cannot be considered anything but patho- 

 logical. It has failed under the stresses on it, but the 

 organism which reacted turned a weak dilatation sac into 

 a strong permanent food pouch. The results to the reacting 

 organisms were many. The ingested food became tem- 

 porarily static, was more thoroughly dealt with, and the 

 organism was not continually feeding. Its whole available 



