4 MANUAL OF ZOOLOGY 



distinct species : if, on the other hand, there is a complete 

 series of gradations 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 positions 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 development 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 



