4 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 knowledge; 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, Rondoletius, 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. 
