100 PROTECTIOX OF PLANTS, 1916-17 



It is too much to expect that the layman will adopt the zoologist's 

 viewpoint, and it is not necessary that he should. But it is of vital import- 

 ance that there should be a more widespread diffusion of knowledge of the 

 life histories of the species commonly attacking man and animals, and of 

 the methods by which they are perpetuated and spread. Only through 

 such knowledge can injurious species be combatted, and only through 

 popular education can rational measures of sanitation be enforced. 



That there is need for such education may be readily seen when we 

 consider the evidence which we have as to the medical importance of these 

 forms. It was only a few centuries ago that medical men attributed a 

 large share of human ills to intestinal worms. A relic of the same belief 

 is seen today in the popular inclination to blame most of the disorders of 

 infancy to "worms." 



The recognition of the role played by bacteria as the cause of disease, 

 and the development of the germ theory during the latter part of the past 

 century contributed largely to the swinging away from the belief in the 

 medical importance of the parasitic worms and, as is usual under such 

 circumstances, the pendulum swung too far. For a time, many prominent 

 medical men regarded such parasites as insignificant, or even as innocuous. 



Our views have had to undergo readjustment and while we now laugh 

 at the extreme beliefs of the mediaeval physicians, we nevertheless recognize 

 the danger of the viewpoint which ingores the grosser parasites as causes 

 of disease and even death of their host. There is today no question among 

 students of the subject but that many such parasites must be seriously 

 reckoned with as a menace — direct or indirect — to the individual and to 

 the community. 



The ways in which internal parasites may cause injury and disease to 

 their host are varied. 



1. They may constitute a drain on the nourishment of the infested 

 animal. This, popularly supposed to be their chief action, is in reality 

 very subordinate. Even when they occur in large numbers it is not 

 possible to explain the effects produced. as merely due to malnutrition. 



2. Mechanical disturbance, due to large numbers of the worms, or 

 to their presence in more limited numbers in certain organs, may cause 

 serious injury or even death of the host. Thus, a common roundworm of 

 poultry occasionally occurs so massed as to plug the intestines of the bird. 

 Similar difficulties have been caused by the large round worm,' Ascaris 

 lumhricoides, in man. A single cyst of the gidworm, Multiceps multiceps, 

 in the brain of a sheep may cause the animal's death. 



