PHYSIOLOGICAL 443 



members of the race who did not vary in the direction of immunity 

 would be gradually eliminated. 



CLASSIFICATION OF POISONS.— No hard-and-fast line can 

 be drawn between poisons and other substances. For there are 

 many chemically diverse kinds of poisons, and even if we say that 

 a poison is defined by its destructive or disintegrative effect when 

 introduced into a living creature, we have to admit that one man's 

 food may be another man's poison, and that much depends on the 

 dose. Common salt is necessary for health, but an enormous dose 

 would be fatal. Sugar also may be a poison. Many a powerful poison, 

 such as strychnine, may be life-saving in a minute dose. An adder's 

 venom will kill a rabbit, but the hedgehog is unaffected. The ovaries 

 of the sea-urchin are often eaten along the Mediterranean coasts, 

 but they produce symptoms of poisoning if used just before they 

 are about to discharge the eggs. There are strange stories of ducks 

 being poisoned by swallowing ripe earthworms. 



The poisoning effect has two sides; it often varies with the 

 physiological state of the assailant or poisoning animal, and also 

 with the condition of the victim or poisoned animal. It may vary 

 with the moon. A strawberry is poisonous to some people, and 

 lobster to others. The hombill enjoys the seeds of nux vomica; and 

 while swallowing one blister-beetle would probably be fatal to a 

 man, the scorpions suck their juices with impunity. 



All living matter, as we have explained, is made up of the nitro- 

 genous carbon compounds called proteins, such as the albumin of 

 white of egg, or the casein of milk ; yet each distinct t5rpe of animal 

 has specific proteins peculiar to itself. And these proteins, for some 

 reason or other, are not good mixers. If the serum of the blood of 

 a particular animal is introduced into the blood of another animal, 

 even a related species, it brings about a destruction of red blood 

 corpuscles. The strange protein has a disintegrative influence, even 

 on substances not very unlike itself. In other words, the strange 

 protein acts as a poison. 



But as man is continually using various proteins as food, and 

 experimenting with proteins that he never tried before, we have to 

 recognise the value of those digestive processes that break down the 

 proteins into their constituents called amino-acids. The smaller 

 molecules that result from the breaking-down of larger ones are able 

 to pass more readily through the wall of the food-canal into the 

 absorbing blood-vessels, and that is great gain; but another aspect 

 of the process is that the edge is taken off the strange proteins. 

 They are disarmed, as it were ; more precisely, they become harmless 

 by being broken up into their constituent amino-acids. Thus we can 

 understand how man can profitably eat many substances which 

 would kill him if introduced into his blood. Not that the blood gives 



