XXVI INTRODUCTION. 



mates, and each may perhaps have some . of different 

 sorts, and in diverse categories. 



But whence come those disgusting beings, whose 

 name alone inspires us with horror, and which instal 

 themselves without ceremony, not in our dwellings, but 

 in our organs, and which we find it more difficult to 

 expel than rats or mice ? They all derive their existence 

 from their parents. 



The time has passed when a vitiated condition of the 

 humours, or the deterioration of the parenchyma was 

 considered a sufficient cause for the formation' of para- 

 sites, and when their presence was regarded as an 

 extraordinary phenomenon resulting from the morbid dis- 

 positions of the organism. "We have reason to hope that 

 this language will, during the next generation, have 

 entirely disappeared from works on physiology and 

 pathology. Neither the temperament nor the humours 

 have any influence on parasites, and they are not more 

 abundant in delicate individuals than in those who enjoy 

 the most robust health. On the contrary, all wild 

 animals harbour their parasitical worms, and the greater 

 part of them have not lived long in captivity, before 

 nematode and cestode worms completely disappear. It 

 is only the imprisoned parasites which do not desert 

 them. 



All these mutual adaptations are pre-arranged, and 

 as far as we are concerned, we cannot divest ourselves of 

 the idea that the earth has been prepared successively 

 for plants, animals, and man. When God first elaborated 

 matter, He had evidently that being in view who was 

 intended at some future day to raise his thoughts to 

 Him, and do Him homage. 



