PARASITES AND PARASITOSIS OF THE 

 DOMESTIC ANIMALS 



PART I 



PRELIMINARY CHAPTERS 



THE EXTERNAL PARASITES 



CHAPTER I 

 INTRODUCTION 



The earth's vast laboratory of living matter includes a flora and fauna 

 in which all of the highly diversified forms encounter conditions operating 

 to restrict their multiplication and to govern the predominance of cer- 

 tain forms over others. These conditions are constituted, first, by 

 topographic and cHmatic variations rendering certain localities more or 

 less inhospitable to some organisms, while others may be uninfluenced or 

 perhaps benefited. Second, there is the behavior of living things toward 

 one another; this may be relatively harmonious or there may be an 

 intense rivalry in which organisms encroach or prey one upon the other, 

 the least fit for the strife being driven to less favorable habitats, progres- 

 sively dwarfed, or ultimately becoming extinct. Though most of these 

 inhibitive influences are not apparent to cursory observation, they are, 

 nevertheless, numerous and varied as well as constant in their operation, 

 constituting a prime factor in the evolution and specialization of organic 

 forms. 



There is, then, a perpetual struggle for existence, which may lead to 

 the seeking of shelter from the conflict in a changed and often degenerate 

 mode of life to which the organism becomes adaptively modified. Thus, 

 through such influences, a terrestrial animal may be driven to an ar- 

 boreal, or even an aquatic or semiaquatic, existence. A defenseless little 

 member of the Insectivora burrows and becomes subterranean, while 

 another finds protection in the nocturnal habit; others seek the shelter 

 of caves or rock crevices, and we often find creatures, usually somewhat 

 ■degenerate, in places which seem to us quite unfavorable to their sup- 

 port. While in such cases the animal continues to lead a free and in- 



