124 ESSAY ON CLASSIFICATION. Part I. 



Rafflesia, and many Orchidea? may be quoted as equally remarkable examples of 

 parasitism. 



Tliere exists the greatest variety of parasites among animals. It would take 

 volumes to describe them and to write their history, for their relations to the 

 animals and plants upon which they are dependent for their existence are quite 

 as diversified as their form and their structure. 



It is important, however, to remark, at the outset, that these parasites do not 

 constitute for themselves one great division of the animal kingdom. They belong, 

 on the contrary, to all its branches ; almost every class has its parasites, and in 

 none do they represent one natural order. This fact is very significant, as it shows 

 at once that parasitism is not based upon peculiar combinations of the leading 

 structural features of the animal kingdom, but upon correlations of a more specific 

 character. Nor is the degree of dependence of parasites upon other organized 

 beings equally close. There are those which only dwell upon other animals, while 

 others are so closely connected with them that they cannot subsist for any length 

 of time out of the most intimate relation to the species in which they grow and 

 multiply. Nor do these parasites live upon one class of animals; on the contrary, 

 they are found in all of them. 



Among Vertebrata there are few parasites, properly speaking. None among 

 Mammalia. Among Birds, a few species depend upon others to sit upon their 

 eggs and hatch them, as the European Cuckoo, and the North American Cowbird. 

 Among Fishes, some small Ophidiums (Fierasfers) penetrate into the cavity of the 

 body of large Holothuria) in which they dwell.^ Echeneis attach themselves to 

 other fishes, but only temporarily. Among Articulata, the number of parasites is 

 largest. It seems to lie in the very character of this type, so remarkable for the 

 outward display of their whole organization, to include the greatest variety of 

 parasites. And it is really among them, tliat we observe the most extraordinary 

 combinations of this singular mode of existence. 



Insects, in general, are more particularly dependent upon plants for their sus- 

 tenance than herbivorous animals usually are, inasmuch as most of them are 

 hmited to particular plants for their whole life, such as the Plant-lice, the Coccus, 

 the Gall Insects. In others, the larva3 only are so limited to particular plants, while 

 the larvfe of others still, such as the Bots, grow and undergo their development 

 under the skin or in the intestines, or in the nasal cavities of other animals. The 

 Ichneumons lay their eggs in the larvje of other insects, upon which the young 

 larvae prey until hatched. Among perfect Insects, there are those which live only 

 in community with others, such as the Ani>Hill Insects, tlie Clavigers, the Clerus, 



^ See above, p. 74, note. 



