4 THE NATURAL HISTORY OF THE COMxMON 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. 



