1222 LIFE : OUTLINES OF GENERAL BIOLOGY 



The microbe of the plague is at home in the rat, and naturally in 

 the rat-flea. When a well-infected rat-flea leaves a dead rat and with 

 its fouled mouth-parts bites man, the plague bacillus continues on 

 its dread journey. But the more cats, the fewer rats, the fewer rat- 

 fleas, and the less plague. It is said that the plague often begins 

 in a mill, in the yard of which there are rats, attracted by the 

 crumbs left by the simple dinners. Hence the common sense pre- 

 ventive suggested, that if there were a dove-cot in the yard, the 

 pigeons would dispose of the crumbs, the rats would not be there, 

 the workers would not be bitten by fleas, and the plague would 

 slumber. The extensive keeping and feeding of pigeons and other 

 birds — even to peacocks in vast numbers in some places — in Indian 

 cities has thus its reward in public health. 



Heredity and Medicine. — In the time of Charles I there was 

 a Frenchman called Jean Nougaret, who suffered from an eye defect 

 known as night-blindness, implying an inability to see in dim light. 

 Now, the lineage of the family has been carefiUly kept since the 

 seventeenth century, and we know that the peculiarity has cropped 

 up in every generation in a certain number of the members. The 

 peculiarity is a "unit character", either there or not there, not 

 blending or fractionating, exhibiting Mendelian inheritance. If a 

 member of the family who was normal married an outsider who 

 was normal, none of their progeny were night-blind. But if an 

 affected member married a normal outsider, the peculiarity cropped 

 up in a certain proportion of the progeny. 



Many instances of unit characters are known in man which 

 illustrate Mendelian inheritance — the colour of the eye, the crinkli- 

 ness of the hair, having "fingers all thumbs", "congenital cataract", 

 and so on; and it is not too much to say that the attitude of 

 medical science to the problems of heredity and disease has been 

 profoundly influenced by recent zoological studies on the organic 

 relation between successive generations. 



Nurture and Medicine.^ — Of great interest also to medicine are 

 the zoological facts in regard to the potency of nurture, {a) Tadpoles 

 fed on minced thyroid gland go on developing, but stop growing. 

 They become differentiated dwarfs; but those fed on a thymus 

 preparation grow without much differentiation, {b) White rats 

 exercised for 90-180 days (which would correspond to many years 

 in man's life), showed an increase in weight by about 20 per cent, 

 as compared with non-exercised rats similarly fed; even the brain 

 shows an average increase of about 4 per cent. — a fact for the con- 

 sideration of athletic clubs, (c) If the newt Proteus remains in the 

 darkness of the caves it develops no pigment, and its eye is a mere 

 vanishing point; if it be kept in the light it puts on dark pigment; 

 if the larva be reared under red light, its eye enlarges and shows 

 some differentiation. These are just three straws which indicate 



