I'AEASITISM IN PLANTS AND ANIMALS. 281 



gradations — instances in which the nutrition may be carried on 

 partly by absorption from the juices of the host, and partly from 

 extraneous sources. That curious association of different forms 

 of life, where two species of animals are constantly found liv- 

 ing together, and evidently, in some unexplained way, helpful 

 to each other, has been called by Professor Van Beneden, com- 

 mensalism or, more recently, by the perhaps preferable term 

 symliosis. Of this condition I shall have to speak more fully 

 further on. 



Looking upon this all-pervading mass of parasitic life, one is 

 apt to think — What is the cause of it all ? how and why did it 

 come into being ? as to the use of it, — that is a knotty question. 

 By what standard are we to measure use ? We are too liable to 

 think as if we ourselves — the human race — were the sum and 

 crown of creation, to which every other thing has to minister. 

 But we are only one item in the great crowd of life ; we have 

 fought our way upward from a poor estate, and still have to 

 do battle against all manner of adverse circumstances — parasites 

 among the rest. So that in this way, at least, parasitic life is 

 an aid to our development and progress. No organisms can 

 advance socially, morally, or physically without waging a con- 

 tinual war against evil. A life of lassitude and inactivity is 

 death. And have we not here part, at any rate, of the answer 

 to the oft-repeated question of the origin of evil. Evil is but 

 comparative good, — the good of to-day may be, and must be, if 

 development is to go on — the evil of to-morrow. 



As to how parasitic life arose and flourished, the answer is 

 tolerably clear. It is simply a result of that fierce struggle for 

 existence which results in the filling up of every available chink 

 or cranny or vacant spot in which life can find a foothold. To 

 begin with, some creature on the verge of starvation or suicide, 

 pursued by bloodthirsty enemies, or from some cause or other 

 "between the devil and the deep sea," has found itself, acci- 

 dentally no doubt, in a position which afforded it shelter and 

 sustenance. May be a small crustacean has been driven into the 

 gill-chamber of some fish, or the egg of some worm has been 

 swallowed by a creature in whose alimentary canal it has found 



