THE STUDY OF ANIMALS ii 



determined, not by a boundary line without, but by a central point 

 within ; not by what it strictly excludes, but by what it eminently 

 includes; by an example, not by a precept; in short, instead of a 

 definition we have a type for our director" (Whewell). Cuvier 

 was the first to recognize the fact that the different groups of 

 animals are related to one another like the branches of a tree, and 

 not like successive portions of a straight line. He recognized four 

 great branches — Vertebrates, Molluscs, Articulates, and Radiates, 

 each distinguished by its special type of structure. 



Since Cuvier 's time it has come to be more and more 

 recognized that in a " natural " classification, i.e. one having 

 regard to as many distinctive features as possible, in contradis- 

 tinction to an " artificial " system which regards only one or a 

 few arbitrarily chosen characters, the groups of animals must 

 be represented diagrammatically by a tree, in which the large 

 branches represent the large groups, their subdivisions the smaller 

 groups, and the leaves individuals. This conclusion has been 

 greatly supported and extended this century by the study of 

 extinct animals, placed on a scientific basis by Cuvier himself; 

 and by the teachings of development or embryology, a zoological 

 branch of which the foundations were firmly laid by von Baer 

 (1792-1876). 



The real meaning of this tree-like arrangement of organisms 

 was at first a mystery, but, thanks more particularly to Charles 

 Darwin and his contemporary Alfred Russell Wallace, who is 

 fortunately yet among us, we now understand it to be an ex- 

 pression of an actual blood-relationship — to be, in fact, a veritable 

 genealogical tree, the outcome of a process of evolution. And 

 hence classification at the present time is a " classification by 

 pedigree ", and is perfect, in so far as we are successful in arranging 

 animals according to their actual affinities. 



3. Zoology as regarded by the Morphologist or Student of 

 Animal Form. — One of the main subdivisions of modern zoology 

 is the branch dealing with the shape or form of animals, using 

 these words in their broadest sense. This part of the subject is 

 Animal Morphology (Gk. morphe, form; logos, a discourse); and 

 it deals not only with the external characters, the outward form, 

 but also with the structure or inward form, as revealed by dis- 

 section and by the use of the microscope. The external characters 

 of animals were naturally those first noticed ; then came the study 



