CHAPTER III. 



THE THEORY OF THE OEIGIN OF PAEASITES 

 REGAEDED HISTOEICALLY. 



Wbee the parasites infesting animals confined to those whicli are 

 temporary and external, their origin and descent woiild present no 

 difficulties to the observer. But numerous forms are found deep 

 in the interior of living bodies, in the brain, kidneys, and other 

 apparently inaccessible organs. It is very surprising, when we 

 expect to meet with only the blood, nerves, and other constituent 

 tissues of the body, to iind independent living animals, frequently 

 of large size, which have left no trace to show how they 

 reached their dwelling-place, and, indeed, are often incapable cf 

 moving about. Under these circumstances, we can easily understand 

 that the presence of parasites has an unusual interest, and that their 

 origin was one of the subjects most eagerly investigated by biologists. 

 The importance of parasites from a medical as well as from a 

 zoological point of view, caused both physicians and naturalists to 

 examine more closely into these facts, which appeared as mysterious 

 and incomprehensible as the origin of Ufe itself. 



In its most general aspect, the question of the origin of internal 

 parasites can be answered only in two ways; either they originate in the 

 tissues in which they are found, or else reach them from the external 

 world. In the former case, they must be spontaneously generated ; in 

 the latter they may, after the ordinary method, be developed from 

 fertilised ova. Indeed, all the conjectures and hypotheses as to the 

 origin of Entozoa brought forward in former centuries can be reduced 

 to these two theories, though the greatest diversities, depending partly 

 upon the views current at the time, and partly upon individual 

 opinions, are to be seen in the way in which these theories were 

 stated. Where facts are silent, there imagination is the more 

 eloquent ; and it is only in our time that a definite solution has been 

 put forward as to the origin of parasites, which rests at the same time 

 time on a firm basis of fact. 



As long as it was believed that a " generatio wquivoca," or 

 "spontaneous generation," as it is usually termed, was a pheno- 

 menon commonly met with among the lower animals, the orio'in 

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