202 DISCOVERY CH. 



that a disease is just as definitely due to a particular 

 cause, preventible or otherwise, though it may not 

 tread closely upon the heels of action. Ignorance of 

 the law may sometimes be pleaded in a court of justice 

 in palliation of an offence ; but Nature accepts no such 

 excuses and decrees a punishment for every crime against 

 her. She never forgives a fault or extenuates it ; inexor- 

 able is her judgment, and inevitable her sentence, 

 which has often to be suffered not only by the offender 

 but also by his children, even to the second and third 

 generations. 



This is a hard saying ; yet it is true, and there is no 

 escape from what it implies. As we have to subscribe 

 to Nature's statutes, it is desirable that our knowledge 

 of them should be as complete as possible. " Give me 

 understanding and I shall keep thy law ; yea, I shall 

 observe it with my whole heart." Where such under- 

 standing does not exist, disease is regarded either as 

 a demon to be exorcised or an " act of God " for which 

 penitence and prayer are remedies. Ignorance made 

 plague the terror of Europe in the Middle Ages ; science 

 has proved that the disease is due to a bacillus which 

 is conveyed by fleas from rat to rat, and from rats 

 suffering from the disease to mankind. Ignorance 

 ascribed malaria to a miasma or bad air arising from 

 marshy places ; science has shown it to be caused by a 

 micro-organism carried from one man to another by a 

 certain species of mosquito. Ignorance of the cause of 

 yellow fever made the regions around the Caribbean 

 Sea the White Man's Grave, where the risk of death for 

 the visitor was greater than in a battle ; knowledge 

 that the disease is associated with a parasite which is 

 communicated from an infected to a healthy person 

 by the bite of a particular mosquito, has been the 



