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fix'st sights something very staggering in the existence of 

 parasites, animal and vegetable ; by which we mean or- 

 ganisms adapted to live at the expense of other organisms. 

 Our imaginary opponent may well say, Why have horses, and 

 oxen, and sheep, and dogs, and poultry, and even wild birds 

 their several insect plagues, as well as still more hideous tor- 

 mentors of the class Vermes ? Do you know, he may say, the 

 repulsive history of some of the Entozoa ? For instance, how, 

 in the case of the Tape- worms, the egg-stage of these loathsome 

 creatures is adapted to be passed in the alimentary canal of 

 one animal, and the adult form in that of another ? Have 

 you never read of the extraordinary life-cycle of the Flukes, 

 which finally find their way into the livers of sheep, or of 

 the Trichince, which are often fatal to man ? Even the 

 fish swimming in the depths of ocean have their minute 

 Crustaceans clinging to various parts of their bodies — un- 

 bidden and life-long guests. Man himself is liable to be 

 attacked by a great many forms, some of which, however, as 

 the Guinea worm, are, it is true, rare and local. I reply that I 

 am aware of all these facts, and freely admit that the existence 

 of parasites is a very serious problem, and it is one that no 

 one can pretend to have solved satisfactorily. 



It is, therefore, with extreme diffidence that the following 

 considerations are offered : — 



Vegetable and animal parasites can hardly be separated. 

 Now, in the case of Fungi, a class wholly parasitic, we know 

 of at least one useful function. A vast number of minute 

 Fungi are the scavengers of the vegetable world. Whatever 

 falls to the ground in the woods, be it leaf, branch, or tree, 

 is at once attacked by various species, which help to restore 

 it again to its native soil in a form adapted for further use. 

 But on the other side must be placed the terrible havoc 

 caused by those species which attack living plants and ani- 

 mals, and are too familiar to us under the dreaded names of 

 rust, mildew, smut, blight, potato-disease, &c. We must 

 confess our profound ignorance of the benevolent aspect of 

 these inflictions. Possibly they form one of Nature^s stern 

 warnings against over-crowding. She seems to tell us that, 

 if we cover squai'e miles of land with one crop — if we bring 

 together enormous aggregations of one animal — nay, even if 

 we interfere in the balance of life by over-stocking moors and 

 salmon-rivers, wo must expect some of her checks on over- 

 population to make their appearance. This, however, I repeat, 

 is offered as a mere suggestion for what it is worth. A ray 

 of light may be thrown upon animal parasites by the now 

 favourite conjecture that they are not original creations, but 



