216 ENTO-PARASITES AND HYGIENE. 



there the parasite can easily contiDue its migrations and 

 get into the interior of the body — the enfco-parasite is 

 created. 



To repeat, for all such changes great length of time is 

 required ; the transformation from the free-living animal 

 to an ento-parasite is long and slow, and cannot be 

 traversed vs^ithin a few generations. It has not been 

 watched by any human eye, and anybody has the right 

 to ask why the origin of parasites is explained in such a 

 way. Well, though we have not seen the whole process 

 of development, we may observe still now a days certain 

 phases, we might call them halting places, on that long 

 road, which, properly put together, enable us to recon- 

 struct the whole road from the starting point to the end. 

 No great difficulty arises when we try to find out the 

 connection between temporary parasites and their free- 

 living relatives, i. e., those forms from which they de- 

 veloped in the course of time. Not so easy is the task, 

 with the stationary and especially the ento-parasites. 



There are, indeed, some which have free-living relatives 

 of rather similar form and shape ; others no longer 

 show any relation to forms from which they may have 

 taken their origin. In such a case nothing is to be ex- 

 pected from the comparison of those creatures with other 

 animals, and we should know little in this direction if 

 we had not an excellent assistant in embryology, whose 

 doctrines are a most valuable key for the unveiling of 

 hitherto unknown facts. It furnishes in many cases 

 hints concerning relations which could not be discovered 

 by any means in the fully developed animals. 



This importance of embryology for the investigation of 

 the origin of our parasites should be stated most em- 

 phatically; besides, a few words must be said in general 

 on the importance of embryological facts. We know to- 

 day after careful observation that the development of any 

 animal from the egg on to the moment when it repro- 



154 



