INTRODUCTION 3 



species: if, on the other hand, there is a complete series of grada- 

 tions between them, they will be considered to form a single 

 variable species. 



As, therefore, animals have to be distinguished from one another 

 largely by structural characters, it is evident that the foundations 

 of a scientific Zoology must be laid in Morphology, the branch of 

 science which deals with form and structure. Morphology may be 

 said to begin with an accurate examination of the external 

 characters ; the divisions of the body, the number and position of 

 the limbs, the characters of the skin, the position and relations of 

 the mouth, eyes, ears, and other important structures. Next the 

 internal structure has to be studied, the precise form, position, 

 &c, of the various organs, such as brain, heart, and stomach, being 

 made out : this branch of morphology is distinguished as Anatomy. 

 And, lastly, the various parts must be examined by the aid of the 

 microscope, and their minute structure, or Histology, accurately 

 determined. It is only when we have a fairly comprehensive 

 knowledge of these three aspects of a given animal — its external 

 characters, its rough anatomy, and its histology — that we can with 

 some degree of safety assign it to its proper position among its 

 fellows. 



An accurate knowledge of the structure of an animal in its 

 adult condition is not, however, all-sufficient. Nothing has been 

 made more abundantly clear by the researches of the last half- 

 century than that the results of anatomy and histology must be / 

 checked, and if necessary corrected, by Embryology — i.e. by the 

 study of the changes undergone by animals in their develop- 

 ment from the egg to the adult condition. A striking instance is 

 afforded by the common Barnacles which grow in great numbers on 

 ships' bottoms, piers, &c. The older zoologists, such as Linnaeus, 

 grouped these creatures, along with Snails, Mussels, and the like, 

 in the group Mollusca, and even the great anatomical skill of 

 Cuvier failed to show their true position, which was made out only 

 when Vaughan Thompson, about sixty years ago, proved, from 

 a study of the newly hatched young, that their proper place 

 is among the Crustacea, in company with Crabs, Shrimps, and 

 Water-fleas. 



Given a sound knowledge of the anatomy, histology, and em- 

 bryology of animals, their Classification may be attempted — that 

 is, we may proceed to arrange them in groups and sub-groups, 

 each capable of accurate definition. 



The general method of classification employed by zoologists is 

 that introduced by Linnaeus, and may be illustrated by reference 

 to the group of Cats which we have already used in the explanation 

 of the terms genus, species, and variety. 



We have seen that the various kinds of true Cat — Domestic Cat, 

 Lion, Tiger, &c. — together constitute the genus Fclis. Now there 



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