Animals further needed muscles for locomotion and nerves for sensibility.  Hence, says Cuvier, it was necessary that the chemical composition of the animal body should be more complicated than that of the plant; and it is so, inasmuch as an additional substance, nitrogen, enters into it as an essential element; while, in plants, nitrogen is only accidentally joined with he three other fundamental constituents of organic beings—­carbon, hydrogen, and oxygen.  Indeed, he afterwards affirms that nitrogen is peculiar to animals; and herein he places the third distinction between the animal and the plant.  The soil and the atmosphere supply plants with water, composed of hydrogen and oxygen; air, consisting of nitrogen and oxygen; and carbonic acid, containing carbon and oxygen.  They retain the hydrogen and the carbon, exhale the superfluous oxygen, and absorb little or no nitrogen.  The essential character of vegetable life is the exhalation of oxygen, which is effected through the agency of light.  Animals, on the contrary, derive their nourishment either directly or indirectly from plants.  They get rid of the superfluous hydrogen and carbon, and accumulate nitrogen.  The relations of plants and animals to the atmosphere are therefore inverse.  The plant withdraws water and carbonic acid from the atmosphere, the animal contributes both to it.  Respiration—­that is, the absorption of oxygen and the exhalation of carbonic acid—­is the specially animal function of animals, and constitutes their fourth distinctive character.

Thus wrote Cuvier in 1828.  But, in the fourth and fifth decades of this century, the greatest and most rapid revolution which biological science has ever undergone was effected by the application of the modern microscope to the investigation of organic structure; by the introduction of exact and easily manageable methods of conducting the chemical analysis of organic compounds; and finally, by the employment of instruments of precision for the measurement of the physical forces which are at work in the living economy.

That the semi-fluid contents (which we now term protoplasm) of the cells of certain plants, such as the Charoe are in constant and regular motion, was made out by Bonaventura Corti a century ago; but the fact, important as it was, fell into oblivion, and had to be rediscovered by Treviranus in 1807.  Robert Brown noted the more complex motions of the protoplasm in the cells of Tradescantia in 1831; and now such movements of the living substance of plants are well known to be some of the most widely-prevalent phenomena of vegetable life.

Agardh, and other of the botanists of Cuvier’s generation, who occupied themselves with the lower plants, had observed that, under particular circumstances, the contents of the cells of certain water-weeds were set free, and moved about with considerable velocity, and with all the appearances of spontaneity, as locomotive bodies, which, from their similarity to animals of simple organisation, were called “zoospores.”  Even as late as 1845, however, a botanist of Schleiden’s eminence dealt very sceptically with these statements; and his scepticism was the more justified, since Ehrenberg, in his elaborate and comprehensive work on the Infusoria, had declared the greater number of what are now recognised as locomotive plants to be animals.