102 SCIENCE FROM AN EASY CHAIR 



the creatures so marked escape the injury which would 

 be caused them by tentative bites. 



There is a curious variation as to susceptibility on 

 the part of man to poison in the flesh of fishes and 

 shell-fishes when taken by him as food. The word 

 " idiosyncrasy " is applied to such individual suscepti- 

 bility, and is, of course, applicable to the susceptibility 

 shown by some persons to the poison of the American 

 poison-vine, described in the last article, and of others to 

 acute inflammation from the dust of hayfields. Some 

 persons cannot eat lobster, crab, or oysters or mussels 

 without being poisoned in a varying degree by certain 

 substances present in those " shell - fish " even when 

 cooked. Often a " rash " is caused on the skin, and 

 colic. Others, again, cannot eat any fish of any kind 

 without being poisoned in a similar way, or possibly are 

 only liable to be poisoned by grey mullet or by mackerel. 

 The most curious cases of this individual variability are 

 found in the rash and fever caused by the vegetable 

 drug quinine in rare instances, and the violent excite- 

 ment produced in some persons by the usually soporific 

 laudanum. All such cases have very great interest as 

 showing us what a small difference separates an agree- 

 able flavour or a valuable medicine from a rank poison, 

 and how readily the chemical susceptibility of a complex 

 organism like man may vary between toleration and 

 deathly response, without any concomitant indication of 

 such difference being apparent (in our present state of 

 knowledge), in two individuals, to one of whom that is 

 poison which to the other is meat. They also furnish 

 a parallel to that marvellous conversion of " toxin " into 

 " anti-toxin," in consequence of which the blood of an 

 animal injected with small, increasing doses of deadly snake 

 poison or diphtheria poison becomes an antidote to the 

 same poison taken into the blood of an unprepared animal. 



