INTRODUCTION 



IN his ceaseless strides towards a domination of the material 

 world, man has encountered many obstacles. Some of these 

 are palpably of his own making. For many of those pre- 

 sented by his living environment, man is responsible only in 

 so far as he forms a part of the intimately interdependent 

 myriad of organisms which make up that environment. 



In his battle with the elements, he has continually im- 

 proved his condition and he has seemingly mastered many 

 problems which did not even disturb the minds of previous 

 generations. It is in relation to other animals and to plants, 

 more like himself in their plastic constitution and powers of 

 reproduction, that he has so far experienced the greatest dif- 

 ficulty in turning the balance to suit his fancy. 



The great variety and usefulness of domesticated animals 

 and cultivated plants that have been selected and developed, 

 shows that there is much in man's living environment that 

 may be diverted to useful ends. These have been sought for 

 and remain, at least in their present state of improvement, 

 only as unwilling guests, ready to depart, or to revert to 

 their former savage state at the first opportunity. 



Another, and far more extensive series, of unbidden guests, 

 is made up of numerous other animals and plants, some 

 originally associated with the human species, and others 

 attracted to it as the result of changes wrought by civiliza- 

 tion. The organisms of this class consist of various disease- 

 causing microbes and parasites, and of living things that 

 either find a more congenial environment where the face of 

 nature has been altered, or have actually been transferred to 

 parts of the world where they did not formerly exist in the 

 orderly arrangement of nature. 



This great train of undesirable animals and plants is con- 

 tinually augmented by new arrivals, mainly through exten- 



