ORIGIN OF INTESTINAL WORMS. 7 



So affording another mode of entrance into men and animals, 

 by the water which the latter drink. 



Many of the young intestinal worms, more or less developed, 

 but still enclosed in the egg-shell, remain quite inactive in their 

 wanderings, and for these passive emigrants it is, of course, a 

 mere matter of chance whether they reach their goal or not. 

 The young of others, having previously left their egg-shell, may 

 take an active share in the process, creeping up out of their holes 

 and corners in wet weather, or in the damp mornings, upon the 

 slippery plants, and so entering the animals fitted for their habita- 

 tion, when they come to seek for food. 



According to an old- standing custom which careful shepherds 

 strictly keep up, sheep are never allowed to be driven out in the 

 morning till all the dew is off the grass, nor yet to graze in damp 

 swampy pastures. By this precaution the shepherds unwittingly 

 protect their charges from the attacks of Strongyli and Distomata. 

 It is on like grounds that seasons of wet weather are so frequently 

 fatal to flocks, it being then easier for the young intestinal worms 

 to enter the sheep and give rise to entozoic pestilence; whilst 

 in continuously dry and hot seasons a great number of these 

 young worms must be dried up and destroyed, and thus the sheep 

 are delivered from their attack and all its evil consequences. 



But in thus expressing my opposition to the various hypotheses 

 of the origin and multiplication of the parasitic worms, it might 

 appear as if I had fallen into the very error I condemn, and the 

 objection might be raised that the explanation I have just given 

 of the singularities observed in the mode of occurrence of the 

 intestinal worms is, like former hypotheses, merely imaginary, 

 and that I am unable to support it by demonstrative experimental 

 evidence. 



This I must beg leave to deny. It is true that what I have 

 said respecting the origin of the Strongylus filaria and of the 

 fluke (Distomwn hepaticum) in sheep, is as yet only an assumption, 

 and not to be regarded as directly proven. Nevertheless, my 

 assumption rests upon the analogy of reliable facts, which I have 

 established by observation in other intestinal worms. The 

 recognition of definite, though at first isolated truths, has often 

 done much for science, since by careful application of the laws 

 of analogy they have furnished the key to phenomena long hidden 

 in obscurity. 



In order to show that emigration and immigration are regular 



