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THE NATURAL HISTORY OF THE COMMON CRAYFISH.

The Biological Sciences embody the great multitude of truths which have been ascertained respecting living beings; and as there are two chief kinds of living things, animals and plants, so Biology is, for convenience sake, divided into two main branches, Zoology and Botany. Each of these branches of Biology has passed through the three stages of development, which are common to all the sciences; and, at the present time, each is in these different stages in different minds.

Every country boy

possesses more or less information respecting the plants and animals which come under his notice, in the stage of common knowdedge;

a good

many persons

have

acquired more or less of that accurate, but necessarily incomplete and unmethodised knowledge, which is under¬ stood by Natural History; while a few have reached the purely scientific stage, and, as Zoologists and Botanists, strive towards the perfection of Biology as a branch of Physical Science. Historically, common knowledge is represented by the allusions to animals and plants in ancient literature; while Natural History, more or less grading into Biology, meets us in the works of Aristotle, and his continuators in the Middle Ages, Bondoletius, Aldrovandus, and their contemporaries and successors.

But the conscious at¬

tempt to construct a complete science of Biology hardly dates further back than Treviranus and Lamarck, at the beginning of this century, while it has received its strongest impulse, in our own day, from Darwin.