The next point which demands notice relates to the nature of the differences between one animal and another, and the question is one of the highest importance. Every animal - as every plant - may be regarded from two totally distinct, and, indeed, often apparently opposite, points of view. From the first point of view we have to look simply to the laws, form, and arrangement of the structures of the organism; in short, to its external shape and internal structure. This constitutes the science of morphology (μoρФή), form, and λoyoς, discourse). From the second, we have to study the vital actions performed by living beings and the functions discharged by the different parts of the organism. This constitutes the science of physiology.

A third department of zoology is concerned with the relations of the organism to the external conditions under which it is placed, constituting a division of the science to which the term "distribution" is applied.

Morphology, again, not only treats of the structure of living beings in their fully-developed condition (anatomy), but is also concerned with the changes through which every living being has to pass before it assumes its mature or adult characters (embryology or development). The term "histology" is further employed to designate that branch of morphology which is specially occupied with the investigation of minute or microscopical tissues.

Physiology treats of all the functions exercised by living bodies, or by the various definite parts or organs, of which most animals are composed. All these functions come under three heads: - 1. Functions of Nutrition, divisible into functions of absorption and metamorphosis, comprising those functions which are necessary for the growth and maintenance of the organism. 2. Functions of Reproduction, whereby the perpetuation of the species is secured. 3. Functions of Correlation, comprising all those functions (such as sensation and voluntary motion) by which the external world is brought into relation with the organism, and the organism in turn reacts upon the external world.

Of these three, the functions of nutrition and reproduction are often collectively called the functions of organic or vegetative life, as being common to animals and plants; while the functions of correlation are called the animal functions, as being more especially characteristic of, though not peculiar to, animals.