The same variety of function is acquired by the ear in its development from its simplest to its most complex form. In the higher animals, the organ of hearing is formed of three parts, an external, middle, and internal portion; but in birds the external ear is wanting; in fishes both the external and middle parts are wanting; in mollusks it is reduced to a simple sack of microscopic dimensions, filled with a liquid in which there are otolithes, or pebbly substances. Such an organ can distinguish noises only; it can recognize nothing of the infinite variety of articulations, notes, tones, melodies, harmonies of the human voice and of musical instruments. There is even a great difference between the disciplined, and therefore differentiated ear of a cultured person, and the undisciplined, and therefore less differentiated ear of a boor. Similar specializations of structure and function pertain to the other senses; but we may pass them. The digestive, circulatory, and respiratory systems, and all the other systems of the animal structure, evince the same law. The lowest form of the circulating fluid, as in sponges, is simply water containing gases and organic particles; and this can scarcely be spoken of as circulating, for it is merely drawn in and then expelled. A little higher in the scale naturalists find a 'chylaqueous fluid,' which oscillates in the general cavity of the sack-like animal. The true blood is another step in development; and even this organized fluid changes its character as the scale advances. Most animals have no heart; and when the organ does first appear, it is but a simple, rudimentary structure, very unlike the complex machine which plays at the centre of circulation in the higher types. Though fishes breathe through their gills, receiving all the oxygen they require from the small amount of air in the water, the swimming bladder is in them the rudimentary lung--a very simple structure, indeed, when compared with the more complex arrangement for respiration in the higher animals. Some animals of gelatinous, and therefore flexible structure, perform digestion by folding their bodies over the food, and pressing the nutritious matter out of it: they extemporize a stomach for the occasion. And even in some of their higher types, in such as have a permanent mouth and stomach, the digestive process is simply a squeezing out of the elements of nutrition. The digestive apparatus, from being a simple sack in the polype and similar organisms, becomes, by a continuous unfolding, the complicated structure which we find in the higher animals, with various organs effecting various parts of the digestive change, and even different parts of the same organ having specialized functions to perform.