ruct and run the railroads, and control the growth of the cities
according to their fancy; and, lastly, how it would be learned that it
is the human being alone that grows and multiplies and that all else is
the result of his activities. Such a supramundane observer would find
himself entering into a new era, in which all his previous knowledge
would sink into oblivion.

Something of this same sort of revolution was inaugurated in the study
of living things by the discovery of cells and protoplasms. Animals and
plants had been studied for centuries and many accurate and painstaking
observations had been made upon them. Monumental masses of evidence had
been collected bearing upon their shapes, sizes, distribution, and
relations. Anatomy had long occupied the attention of naturalists, and
the general structure of animals and plants was already well known. But
the discoveries starting in the fourth decade of the century by
disclosing the unity of activity changed the aspect of biological
science.

==The Cell Doctrine==.--The cell doctrine is, in brief, the theory that
the bodies of animals and plants are built up entirely of minute
elementary units, more or less independent of each other, and all
capable of growth and multiplication. This doctrine is commonly regarded
as being inaugurated in 1839 by Schwann. Long before this, however, many
microscopists had seen that the bodies of plants are made up of
elementary units. In describing the bark of a tree in 1665, Robert Hooke
had stated that it was composed of little boxes or cells, and regarded
it as a sort of honeycomb structure with its cells filled with air. The
term cell quite aptly describes the compartments of such a structure, as
can be seen by a glance at Fig. 7, and this term has been retained even
till to-day in spite of the fact that its original significance has
entirely disappeared. During the last century not a few naturalists
observed and described these little vesicles, always regarding them as
little spaces and never looking upon them as having any significance in
the activities of plants. In one or two instances similar bodies were
noticed in animals, although no connection was drawn between them and
the cells of plants. In the early part of the century observations upon
various kinds of animals and plant tissues multiplied, and many
microscopists independently announced the discovery of similar small
corpuscular bodies. Finally, in 1839, these observati