ANIMAL BIOLOGY. [Chap. I. 



set of problems connected with functions and the chemical or 

 r^etabolic changes which accompany the exercise of function. 

 These fall within the department of Physiology. And just as 

 there is a morphology of organisms, of organs, and of tissues, so 

 too is there 



1. The physiology of organisms. 



2. The physiology of organs. 



3. The physiology of tissues and tissue cells. 



Other biological problems fall under the head of Distribution. 

 Frogs do not occur all the world over. They have a definite 

 distribution in space. Nor do they occur in all geological 

 strata. They have a definite distribution in time. Hence the 

 problems connected with the distribution of animals fall 

 naturally under two heads : 



1. The distribution in space (chorology). 



2. The distribution in time (chronology). 



Finally, there is a fourth department of Animal Biology, which 

 deals with the causes of those facts which are studied in the 

 other three departments. This is of recent growth. To the 

 questions, What are the structures and functions of tissues, 

 organs, and organisms ? What is the series of developmental 

 changes undergone 1 What is the distribution in space and time? 

 is added a further question, How has all this been brought 

 about 1 And thus to the three departments of Biology which 

 deal with the facts of structure, the facts of function, and the 

 facts of distribution, is added a fourth, called ^twlogy, which 

 deals with the causes to which these facts are due. And this 

 fourth department is co-extensive with the other three, which 

 may be grouped together under the head of Descriptive Biology. 

 Descriptive Biology thus deals with facts, and Etiology with 

 causes. And since no fact can be said to be understood until 

 we have some knowledge of its manner of causation, it is clear 

 that ^Etiology is necessary to and supplementary to the other- 

 wise incomplete science of Descriptive Biology. Descriptive 

 Biology without ^Etiology lacks life : ^Etiology not founded on 



