Parasitism and Social Reciprocity. 193 



tions of the host and so alters the intimate character and constitution of 

 the tissues themselves or causes fermentation of the blood, that a new 

 life and unnatural energy are infused into the cells composing them, by 

 which they consume all the nutriment appropriate to the growth of the 

 parasite, and so exclude it. We are to remember that every tissue cell 

 of the body is a separate animal in precisely the same sense in which a 

 germ or spore of vaccine or small-pox is, or even a tape worm or a sac- 

 culina, the only difference consisting in the degree of degradation to 

 which they have severally come by reason of the disuse of functions. 

 The competition then comes between the normal tissue cell stimulated 

 to preternatural energy and the adventitious parasitic cell. During its 

 season of stimulated activity the parasite is unable to gain a footing. 

 This season of the repellant energy of the tissue cell is of a definite du- 

 ration, after which the cell relapses to its original mode of unstimulated 

 life. As against small-pox it is said to last seven years, usually, but is 

 good against measles for a whole lifetime. 



The fact that we '.'outgrow" tendencies to various certain sorts of 

 disease appears to countenance this theory, for this "outgrowing " can 

 be nothing more than such change in the constitution of the cells of the 

 body as to give them a more pronounced individuality and aggressive 

 self-assertion, consolidation of texture and vigor of vitality, by which 

 they are able to draw for their own support, to the exclusion of parasitic 

 disease-producing germs or other functional disturbance. Parasitic life 

 may therefore be called a struggle between two modes of vitality 

 through two sets of organic cells, the one adventitious and hungry, and 

 the other normal to the place and conservative, both so far degenerated 

 as to be unable to elaborate for themselves a supply of food, and 

 depending upon a supply to be furnished by a third agency of cells, 

 which being a definite and limited supply, may prove insufficient for 

 both. This is obviously true in the case of the tape worm, which ab- 

 sorbs an immense proportion of the nutritive matter required by the 

 general tissues of the body. In many cases the parasite draws upon 

 special parts and selects special nutriment to the damage of a special 

 function of the host. 



When the Rhizocephala (saculinas) infest the hermit crab (Pagurus^ 

 the female germ glands are never developed in the host. The growth 

 of the host is not stopped but its breeding powers are. The larvae of 

 Trematoda, boring worms, infest the water snail, Lymnea stagnalis, and, 

 in like manner, destroy its fertility without apparently affecting its gen- 

 eral health in other respects. This is very curious when it is considered 

 that the plundered nutriment goes almost exclusively to the support and 

 development of the eggs and young of the parasite, which thus perpetu- 

 ates his own worthless stock by the very means that should give a prog- 



