INTRODUCTION. 
In considering the systematic arrangement of any class of organized beings, the further the 
reseaiches are carried, and the more closely the phenomena are studied, the more hopeless 
does the piospect appear of assigning any fixed limits to the minor groups of forms, which 
blend into one another here and there with a perfection of harmony so true, that the point 
of contact seems altogether lost; this, indeed, forms one of the strongest presumptions in 
favour of the adoption, in some form or other, of a theory of evolution as the basis on which 
to conduct inquiries ; but the questions, what hypothesis shall be adopted ? and, wherein 
lies the limit of affinities'? are still wrapt in so much obscurity, that in treating of minor 
groups like the one under consideration it is perhaps preferable to adhere to a simple 
classification based on external resemblances, and arranged solely with a view to facilitate 
identification of species. 
Though the introduction to a monograph of a single family is necessarily too limited a 
space m winch to enter on a subject embracing the entire animated kingdom, and any dis- 
cussion of the evidence on which the hypotheses are based must, under the circumstances, be 
bnef, it is m the hope of calling attention to some points which are to many minds obscure, 
and stimulating discussion on them, that this chapter has been written. 
_ r ^ ie i nc l u i r i es concerning the origin of the almost countless species into which the indi- 
viduals composing the animated kingdom past and present have been ultimately divided, are 
justly based on the assumption, that, as perfect order and regularity of laws have been found 
to govern the inanimate universe, so, in a similar manner they pervade also the animated 
world, and that, if these laws were fully known, the rise of a new genus or species, and its 
prominent characteristics, might be as readily calculated as the advent of a comet or the 
eclipse of a planet. As yet the only laws of which we have any definite conception, are 
those of geneiation and death: and these, with certain modifications, have been held by the 
highest modern authorities to be sufficient of themselves to account for all the phenomena 
of natural life. It appears to be a fact that, notwithstanding the close resemblance to 
parents which the law of generation entails, no two individuals even under the most favour- 
able . circumstances are ever precisely alike; and that if a natural tendency is assumed to 
multiply these differences in the required direction, they are sufficient to produce specific 
distinctions, has been taken as the basis of an evolution theory. 
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