5 2 THE OLIVE LEAF. CHAP. 



selves and prey upon others; how parasitism inevit- 

 ably reduces form and function alike in the mental 

 and spiritual as in the physical world to the lowest 

 and basest point. No creature can abdicate its glorious 

 individuality and independence with impunity. Nature 

 stamps upon it her hall-mark of degradation. 



But there is another principle besides parasitism 

 throughout nature, viz., commensalism. The term was 

 applied, in the first instance, to a class of humble 

 animals living in the sea that are taken under the 

 protection of a higher class, feed at the same table, 

 and associate together in the various pursuits of 

 life, but have otherwise no connection with each 

 other. The commensals partake of the same food 

 with their hosts, but they do not feed upon them, 

 do not make use of the pabulum that has been 

 organized for them by passing through the system of 

 their associates. There is a large number of these 

 curious creatures, whose habits are exceedingly interest- 

 ing and instructive. But the term that describes them 

 might be extended so as to include plants as well as 

 animals, on land as well as in the sea, which exhibit 

 similar peculiarities. Ferns, lichens, and mosses are 

 not parasites, for they do not injure the trees upon 

 which they grow. They feed upon the same air and 

 sunshine, and imbibe the same moisture, but they pro- 

 cure their own living independently of the structures 

 which give them support and protection. So, too, with 

 the ivy, for though it attaches itself to the bark of a tree 

 by thousands of rootlets, it does not derive its susten- 



