INTRODUCTION XIH 



cold, light and darkness. We are aided by certain animals 

 and plants, which we have domesticated, that is, trained to 

 tolerate and even enjoy being exploited for our food and 

 service. Other species have not been tamed: hons and tigers 

 are still fierce enemies; plants of the jungle and undomesti- 

 cated herbs which we call weeds fight constantly with us for 

 possession of the land. A notable enemy plant is the prickly 

 pear which is conquering thousands of acres of previously 

 fruitful land every month in Austraha. The microscopic forms 

 of fife, bacteria and protozoa, threaten us today more than 

 the fierce giants of old. Part iv is devoted to this large 

 subject of environment. 



Much of the exact knowledge in medicine and other sciences, 

 reported in Parts in and iv of this book, has been obtained 

 through dissection of human bodies after death and by 

 cautious experiments on Hving beings — scientists often exper- 

 imenting on themselves as in yellow fever and typhus — and 

 equally cautious work on animals. The information obtained 

 from the latter source is usually applicable to human beings, 

 since we have many similarities to other animals. Misguided 

 people called "anti-vivisectionists" have attempted to tie 

 the hands of investigators in the use of animals. Women, 

 wearing furs taken from animals trapped and killed with 

 great cruelty, have often expressed the most sentimental 

 sympathy for animals used under merciful conditions for 

 experimental work. Such people, even when consistent and 

 actuated by the best of motives, are often unaware of the use 

 of anesthetics in animal experiments and of the care with 

 which the research is carried out, and also of the intensity of 

 human suffering which such experiments tend to relieve 

 or prevent. Dr. W. W. Keen has suggested that agitators 

 against animal experimentation should be compelled to 

 watch preventable death with all its grimness in hospitals 

 and homes or be confronted with a dead guinea pig and the 

 dead body of one of their friend's children and be asked to 

 choose between them. 



In the final division, Part v, the future is discussed on the 

 basis of facts presented earlier and of studies of the tendencies 

 of evolution in man and other animals. In this section is 

 included a tentative report on the inheritance of disease 



