XXXVI INTRODUCTION. 



Morphology and Physiology are of equal rank in biological science, and a previous 

 acquaintance with Morphology is assumed as a basis for the comprehension of 

 Physiology, since the work of an organ can only be properly understood when its 

 external form and its internal arrangements are known. Development occupies a 

 middle place between Morphology and Physiology ; it is a morphological discipline 

 in so far as it is concerned with the description of the parts of the developing 

 organism ; it is a physiological doctrine in so far as it studies the activities and 

 vital phenomena during the course of development. 



MATTER. The entire visible world, including all organisms, consists of matter, 

 i.e. j of substance which occupies space. 



We distinguish jwnderable matter which has weight, and imponderable matter 

 which cannot be weighed in a balance. The latter is generally termed ether. 



In ponderable materials, again, we distinguish their form, i.e., the nature of their 

 limiting surfaces; further, their volume, i.e., the amount of space which they 

 occupy ; and lastly, their aggregate condition, i.e., whether they are solid, fluid, or 

 gaseous bodies. 



Ether. The ether fills the space of the universe, certainly as far as the most 

 distant visible stars. This ether, notwithstanding its imponderability, possesses 

 distinct mechanical properties ; it is infinitely more attenuated than any known 

 kind of gas, and behaves more like a solid body than a gas, resembling a gelatinous 

 mass rather than the air. It participates in the luminous phenomena due to the 

 vibrations of the atoms of the fixed stars, and hence it is the transmitter of light, 

 which is conducted by means of its vibrations, with inconceivable rapidity (42,220 

 geographical- miles per second) to our visual organs (Tyndall). 



Imponderable matter (ether) and ponderable matter are not separated sharply 

 from each other ; rather does the ether penetrate into all the spaces existing 

 between the smallest particles of ponderable matter. 



Particles. Supposing that ponderable matter were to be subdivided con- 

 tinuously into smaller and smaller portions, until we reach the last stage of 

 division in which it is possible to recognise the aggregate condition of the matter 

 operated upon, we should call the finely-divided portions of matter in this state 

 jmrticles. Particles of iron would still be recognised as solid, particles of water as 

 fluid, particles of oxygen as gaseous. 



Molecules. Supposing, however, the process of division of the particles to 

 be carried further still, we should at last reach a limit, beyond which, neither by 

 mechanical nor by physical means, could any further division be effected. We 

 should have arrived at the molecides. A molecule, therefore, is the smallest amount 

 of matter which can still exist in a free condition, and which as a unit no longer 

 exhibits the aggregate condition. 



Atoms. But even molecules are not the final units of matter, since every 

 molecule consists of a group of smaller units, called atoms. An atom cannot exist 

 by itself in a free condition, but the atoms unite with other similar or dissimilar 

 atoms to form groups, which are called molecules. Atoms are incapable of further 

 subdivision, hence their name. We assume that the atoms are invariably of the 

 same size, and that they are solid. From a chemical point of view, the atom of 



