ells, each cell having its own life independent of
the others, though influenced by them.

All the morphological characters of animals and plants are the results
of the mode of multiplication, growth, and structural metamorphosis of
these cells, considered as morphological units.

All the physiological activities of animals and plants--assimilation,
secretion, excretion, motion, generation--are the expression of the
activities of the cells considered as physiological units. Each
individual, among the higher animals and plants, is a synthesis of
millions of subordinate individualities. Its individuality, therefore,
is that of a 'civitas' in the ancient sense, or that of the Leviathan
of Hobbes.

There is no absolute line of demarcation between animals and plants.
The intimate structure, and the modes of change, in the cells of the
two are fundamentally the same. Moreover, the higher forms are
evolved from lower, in the course of their development, by analogous
processes of differentiation, coalescence, and reduction in both the
vegetable and the animal worlds.

At the present time, the cell theory, in consequence of recent
investigations into the structure and metamorphosis of the 'nucleus,'
is undergoing a new development of great significance, which, among
other things, foreshadows the possibility of the establishment of a
physical theory of heredity, on a safer foundation than those which
Buffon and Darwin have devised.

[Sidenote: Spontaneous generation disproved.]

The popular belief in abiogenesis, or the so-called 'spontaneous'
generation of the lower forms of life, which was accepted by all the
philosophers of antiquity, held its ground down to the middle of the
seventeenth century. Notwithstanding the frequent citation of the
phrase, wrongfully attributed to Harvey, 'Omne vivum ex ovo,' that
great physiologist believed in spontaneous generation as firmly as
Aristotle did. And it was only in the latter part of the seventeenth
century, that Redi, by simple and well-devised experiments,
demonstrated that, in a great number of cases of supposed spontaneous
generation, the animals which made their appearance owed their origin
to the ordinary process of reproduction, and thus shook the ancient
doctrine to its foundations. In the middle of the eighteenth century,
it was revived, in a new form, by Needham and Buffon; but the
experiments of Spallanzani enforced the conclusions of Redi, and
compelled the advocates