6 PRINCIPLES OF ANIMAL BIOLOGY 



quent intervals or rarely. Supporting evidence of such changes is found 

 in nearly every branch of zoology. Paleontology furnishes such evidence 

 in the differences between animals of successive geological periods. 

 Many facts of distribution are readily explained only by assuming evolu- 

 tion. The study of morphology and physiology by revealing similarities 

 between different animals, indicates their relationship through a com- 

 mon origin, and the whole science of taxonomy has its basis in change 

 with descent. More of the details of this evidence are pointed out in later 

 chapters. Evolution is not, then, an isolated science, but is closely bound 

 up with morphology, physiology, taxonomy, zoogeography, and pale- 

 ontology; for in each of these fields valuable evidence of evolution is found, 

 and the explanation of evolution and evidence of the course which it 

 has taken are important parts of them all. 



Summary and Comparison. — The seven fields of knowledge described 

 in the foregoing paragraphs may be called the zoological sciences. Every 

 fact of zoology may be referred to one or another of these subjects. They 

 are not wholly distinct from one another, however, for not infrequently 

 a single fact belongs to two or more of the sciences. That is, the fields 

 overlap. The merging of evolution into the other problems of paleon- 

 tology, zoogeography, taxonomy, and others has been pointed out. 

 Ecology may very largely deal with physiological relations. Phj^siological 

 facts are in most instances also morphological ones, since it is usually 

 difficult to separate functions from the structures in which they are mani- 

 fested. This overlapping and the merging of one subject into another, 

 far from beings confusingy' is eloquent testimony of the unity of the whole 

 field of zoology. There is also a unity of zoology and botany, which 

 is the science of plants, for in botany the same seven subdivisions 

 exist. There is a morphology, a taxonomy, an ecology, and an evo- 

 lution of plants; and but for the immobihty of most plants and the 

 differences in their modes of hfe, the principles involved are largely 

 the same as for animals. Paleontology and zoogeography have their 

 counterparts in botany, but the study of fossil plants is often called 

 paleobotany while the science of plant distribution is named phjiio- 

 geography. This book, being a general discussion, will contain elemen- 

 tary facts and principles from each of the branches of the science. The 

 student will find it a useful exercise, as he reads its pages, or discovers 

 new things in the laboratory, to stop and reflect which of the divisions 

 of zoology he is for the moment studying. 



Although zoology of today comprises the seven sciences mentioned 

 above, it would be a mistake to suppose that it has always been so in- 

 clusive. Some of the branches are much older than others. Morphol- 

 ogy, as has been stated, is very ancient, while ecology is a mere infant. 

 Physiology has been known for centuries, while the contemplation of 

 evolution has been common for but sixty years. Zoology, then, has a 



