2 GENERAL PRINCIPLES OF ZOOLOGY 



Morphology, or the study of form, has first to describe all which can 

 be seen externally, as size, color, proportion of parts. But since the 

 external appearance cannot be understood without knowledge of the 

 internal organs which condition the external form, the morphologist 

 must make use of dissection, of Anatomy, and describe their forms and 

 methods of combination. In his investigation he only stops when he has 

 arrived at the morphological elements of the animal body, the cells. 

 Therefore we cannot contrast Morphology and Anatomy, and ascribe to 

 the former the description of only the external, and to the latter of only 

 the internal parts; the kind of knowledge and the mental processes are 

 the same in both cases. The distinction, too, is unnatural, since in 

 many instances organs which usually lie in the interior of the body, belong 

 in other cases to the surface of the body, and are accessible for direct 

 observation. 



Comparative Anatomy. For morphology, as for every science, 

 the mere accumulation of facts is not sufficient to give the subject the 

 character of a science; an additional mental elaboration of this material 

 is necessary. Such a result is reached by comparison. The morphologist 

 compares animals with each other according to their structure, in order 

 to ascertain what parts of the organization recur everywhere, what only 

 within narrow limits. He thus gains a double advantage: (i) an insight 

 into the relationships of animals, and hence the foundation for a Natural 

 System; (2) the evidence of the laws which govern organisms. Any 

 organism is not a structure which has arisen independently and which is 

 hence intelligible by itself: it stands in relation to the other members of 

 the animal kingdom. We can only understand its structure when we 

 compare it with the closely and the more distantly related animals, as 

 when we compare man with the other vertebrates and with many lower 

 invertebrates. We have to consider one of the most mysterious phenom- 

 ena of the organic world, the path to the full explanation of which was 

 first outlined by the Theory of Evolution, as will be shown in another 

 chapter. 



Ontogeny. To morphology belongs, as an important integral part, 

 Ontogeny or Embryology. Only a few animals are completely formed 

 in all their parts at the beginning of their individual existence; most of 

 them arise from the egg, a relatively simple body, and then step by step 

 attain their permanent form by complicated changes. The morphologist 

 must determine by observation the different stages, compare them with 

 the mature animals, and with the structure and developmental stages of 

 other animals. Then there is revealed to him the same conformity to law 

 which dominates the mature animals, and a knowledge of this conformity 



