16 THE ANATOMY OF INVERTEBRATED ANIMALS. 



tions, as we employ the names of " electricity " and " electrical 

 force " to denote others ; but it ceases to be proper to do so, if 

 such a name implies the absurd assumption that either " elec- 

 tricity " or "vitality" is an entity playing the part of an effi- 

 cient cause of electrical or vital phenomena. A mass of living 

 protoplasm is simply a molecular machine of great complexity, 

 the total results of the working of which, or its vital phenom- 

 ena, depend, on the one hand, upon its construction, and, on 

 the other, upon the energy supplied to it ; and to speak of 

 " vitality " as anything but the name of a series of operations 

 is as if one should talk of the " horologity " of a clock. 



Living matter, or protoplasm and the products of its meta- 

 morphosis, may be regarded under four aspects : 



(1.) It has a certain external and internal form, the latter 

 being more usually called structure ; 



!2.) It occupies a certain position in space and in time ; 

 3.) It is the subject of the operation of certain forces, in 

 virtue of which it undergoes internal changes, modifies exter- 

 nal objects, and is modified by them ; and 



(4.) Its form, place, and powers, are the effects of certain 

 causes. 



In correspondence with these four aspects of its subject, 

 Biology is divisible into four chief subdivisions I. MORPHOL- 

 OGY; II. DISTRIBUTION; III. PHYSIOLOGY; IV. ^ETIOLOGY. 



I. MORPHOLOGY. 



So far as living beings have a form and structure, they 

 fall within the province of Anatomy and Histology, the latter 

 being merely a name for that ultimate optical analysis of 

 living structure which can be carried out only by the aid of 

 the microscope. 



And, in so far as the form and structure of any living 

 being are not constant during the whole of its existence, but 

 undergo a series of changes from the commencement of that 

 existence to its end, living beings have a Development. The 

 history of development is an accuont of the anatomy of a liv- 

 ing being at the successive periods of its existence, and of the 

 manner in which one anatomical stage passes into the next. 



Finally, the systematic statement and generalization of 

 the facts of Morphology, in such a manner as to arrange liv- 

 ing beings in groups, according to their degrees of likeness, 

 is Taxonomy. 



