Parasitism and Social Reciprocity. 183 



CHAPTER XXV. 



PARASITISM AND SOCIAL RECIPROCITY. 



The term Parasite, made up of two Greek words, Para, alongside, 

 and Sitos, food, may be freely translated to mean one who partakes of 

 the food of another. The Tapeworm is a most complete example of a 

 parasite in this literal arid original sense, because he intrudes himself 

 into the " very kitchen " of his host and helps himself to the victuals as 

 fast as they are prepared, often not even leaving enough to keep his 

 host from starvation. But the use of the term has been extended to 

 include those who do not eat with the host exactly but eat A?'m, or at 

 least such parts of him as can be spared without fatal results, and also 

 still others that neither eat their host nor his food but manage to get a 

 good living out of the refuse of his food, the excreta of wasted tissues, 

 and the accidental dirt that sticks to him. 



A host is necessary to a parasite, and in a great many instances a 

 parasite is necessary to the host. The relationships of these two, the 

 extent to which they reach and the mutual inter-dependence which these 

 relationships have established between the various mundane organisms, 

 are of much greater consequence than one might suppose. To the ex- 

 tent that a host becomes dependent upon the parasite it may be said to be 

 itself parasitical. And if we charge parasitism upon every organism 

 that avails itself, of the labors of others, we shall find such a mutual 

 dependence running through the whole animal kindgdom and crossing 

 over into the vegetable kingdom, that we will have to set down all the 

 animals and a large part of the vegetables in the list of parasites. This 

 mutual dependence grades all the way from that which is- essential to 

 existence itself, to that in which the mutual offices are simply helpful, 

 but which could be got along without. It is evident that the develop- 

 ment of parasitism must have begun at the last end of this grade, the 

 partial end, and the beginning must have been made in the vegetable 

 kingdom. The first organisms got their living directly from the mineral 

 kingdom, and when any of them began to live to any extent upon their 

 fellow organisms it was not a vital necessity, but only n convenience to 

 do so. 



But as their living upon others rendered it unnecessary to use certain of 

 their own powers that had been developed in their struggle with the min- 

 eral kingdom, these powers dwindled with disuse and became aborted, 

 while others better adapting them to profit by their new relationships 

 became developed. So that to-day but very few vegetables exist that 

 would remain Miiything like what they are if transplanted to such an 

 original mineral soil, devoid of all organic matter, as gave sustenance 



