124 ESSAY ON CLASSIFICATION 



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 para- 

 sites do not constitute for themselves one great division of the ani- 

 mal 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 degTee of dependence of parasites 

 upon other organized beings equally close. There are those which 

 only dwell upon other animals, while others are so closely connec- 

 ted 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 Holothuriae 

 in which they dwell. Echeneis attach themselves to other fishes, but 

 only temporarily. Among Articulata the number of parasites is larg- 

 est. 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 that we 

 observe the most extraordinary combinations of this singular mode 

 of existence. 



Insects in general are more particularly dependent upon plants for 

 their sustenance than herbivorous animals usually are, inasmuch as 

 most of them are limited to particular plants for their whole life, 

 such as the Plant-lice, the Coccus, the Gall Insects. In others the 

 larvas only are so limited to particular plants, while the larvae 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 larvx of other insects, 

 upon which the young larvas prey until hatched. Among perfect 

 Insects there are those which live only in community with others, 



