258 PRINCIPLES OF ANIMAL BIOLOGY 



studied. To know which characters best indicate kinship is the chief 

 problem. In the higher ranks of the classification, those qualities which 

 are constantly associated with one another are presumably best. Thus 

 feathers are constantly associated with wings, a beak, claws, a four- 

 chambered heart, and warm blood. These are the marks of one class, the 

 birds. This principle may be pushed down to the lower ranks, the orders 

 and families, but in less marked degree. When it is used for genera and 

 species it is still valid but often difficult to apply. For species the uncer- 

 tainties of its application are so great that some systematists have 

 advocated abandoning it in favor of some more or less arbitrary scheme. 



Relations of Taxonomy. — Classification has wide connections with 

 nearly all other phases of biology. In a practical manner every biologist 

 has occasional or frequent use for the technical knowledge of the systema- 

 tist, and this requirement is not a purely formal one. Many investiga- 

 tions whose principal aim is entirely apart from classification must, 

 nevertheless, constantly use the data of taxonomy. Thus the zoogeog- 

 rapher, as will be apparent in Chap. 21, is not primarily interested 

 in classification ; but in order to discover the principles which have guided 

 migration or determined extinction in the past, he must be thoroughly- 

 conversant with the taxonomy of the group whose distribution he studies. 

 The paleontologist also requires a knowledge of classification not only 

 of extinct forms but of their living relatives. The Work of the physiolo- 

 gist frequently involves the question of relationship, as does that also 

 of the ecologist. Indeed, every biological field is in very close connection 

 with taxonomy. 



This intimate relation is not one-sided, for each of the phases of 

 biology contributes to a knowledge of classification. Distribution and 

 fossil forms supply information where morphology fails or may refute 

 conclusions based on morphology alone. Physiological facts must be 

 taken into account in explaining the formation of species. Ecological 

 relations must be understood if certain taxonomic questions are to be 

 correctly answered. In practice, this close relation between taxonomy 

 and the other phases of biology is not always observed, but all of them 

 suffer from its neglect. 



References 



Gill, T. Systematic Zoology: Its Progress and Purpose. Annual Report of Smith- 

 sonian Institution, 1907. (Pp. 449-471 for history of taxonomy.) 



