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in the same individuals, at different stages of their development, 
is most striking and instructive. 
Again, these developmental stages, the transformations or 
metamorphoses of insects, some knowledge of which is also 
necessary for the comprehension of the classification of these 
animals, furnish a study of never-ceasing interest, partly for its 
own sake, partly as giving the student a clear conception of the 
phenomena of metamorphosis, which plays so important a part in 
other departments of zoology, and partly from the views which 
it opens up as to the natural history of insects and their complex 
relations to the world outside them. Here the parasitism of so 
many insects in their preparatory stages may especially be cited, 
as affording an endless and most instructive subject of investi- 
gation ; and the whole series of phenomena comprised in the 
life- history of insects affords an easily studied representation of 
the great system of checks and counterchecks which pervades 
all nature in the destruction of herbivorous by carnivorous 
animals, of the latter by other carnivores, and of both by para- 
sites. Indeed, no other class of animals exhibits these inter- 
relations and mutual reactions between different organisms so 
clearly and so multifariously as the insects. Besides the ordinary 
division into herbivorous and carnivorous forms, we find many 
of both series restricted to one particular article of diet, or to 
nourishment derived from a very few species nearly allied to 
each other ; in their modes of activity insects reproduce those 
of all other classes of animals, combined with a few peculiar to 
themselves ; the insidious phenomena of parasitism are displayed 
by them with a perfection of distinctness such as we meet with 
nowhere else ; and their influence is exerted in a thousand ways 
for the modification of other organisms with which they are 
brought into contact. Thus, according to Mr. Darwin’s theory, 
which is adopted by a great many naturalists, the action of 
insects is of the utmost importance in the fertilisation of flowering 
plants, — nay, as an extension or corollary of this view, we find 
some who are prepared to maintain that insects are the cause of 
the development and beautiful coloration of flowers. All these 
different aspects of the relations of insects to the world outside 
them open up an infinity of paths for investigation, each of 
them leading to most interesting and important results, and 
calling for an exertion of the powers of observation which, as a 
mere mental training, cannot but produce the most beneficial 
results. Moreover, so much remains to be done in most of these 
fields of research, that almost every earnest worker may look 
forward to the probability of ascertaining some previously un- 
known facts of more or less importance — a hope which is not 
without its influence upon most minds. By the knowledge of 
the facts involved in the recognition of this general system the 
