ON PARASITES. 15 



indefatigable zeal with whicli modern helminthologists pursue tlieir 

 favourite study answer. Consider Eudolpbi who consecrated a life 

 to the collection and classification of parasites ; Bremser who collected 

 in his Atlas the greatest number of facts upon the subject known at 

 his time ; Dujardin who in five years explored 3000 animals in search 

 of parasites ; Leuwenhoeck, who maintained two pediculi in his stock- 

 ings for two months, to ascertain their power of increase. To form 

 the museum of helminthology at Vienna and collect 368 specimens, 

 in five years, forty-five thousand vertebrata were opened. 



Ignoring the prejudices of the vulgar mind, the modern naturalist 

 pushes his researches into the most remote localities, the most for- 

 bidding places, confident that his labours will not be fruitless. Their 

 functions, numbers, history, intimate relation with industrial pur- 

 suits, and with medicine, all combine to give interest to parasites. 



Each tissue in plants and animals seems adapted to support some 

 special inhabitant. Among vegetables, the root, bark, duramen, and 

 above all, the leaves, support a numerous secondary existence ; and 

 animals are equally liable to the encroachment of parasites. One 

 species infests the cellular tissue, another the brain, another the liver, 

 and so on. 



The aphides, from their numbers and peculiar embryology, merit 

 special attention. One species at least, and often several, of these 

 diminutive creatures belong to every species of plant. The sensi- 

 bilities of some of them are so acute that only a single species 

 of plant will serve for their food. Others are not so susceptible, 

 but subsist upon all leaves that they light upon. The procreative 

 powers of these creatures are so enormous, that Eeaumur esti- 

 mated 5,904,900,000, as the possible offspring in the fifth genera- 

 tion from a single aphis. This fecundity sufficiently accounts for 

 the enormous destruction of plants which they yearly inflict. Not 

 unfrequently they have caused such fearful ravages over large regions 

 of country, that governments have adopted compulsory measures for 

 their destruction. They constitute many of the blights spoken of in 

 common parlance. Their embryology as far as I know has no paral- 

 lel among the rest of the insect world, but finds its analogies among 

 the entozoa. 



Provision is always carefully made to keep each species of animal 

 in due bounds. Those creatures that are most liable to destruction 

 have the greatest powers of reproduction. 



