amental 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 c