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As before remarked, reproduction is a function common to all animals
and to all plants. Every organized being has the power to reproduce
itself, or to produce, or aid in producing, other individuals like
itself. It is by means of this function that plants and animals increase
When we consider the great diversity of characters illustrated in
animal and vegetable life, and the infinite variety of conditions and
circumstances under which organized creatures exist, it is not
surprising that modes of reproduction should also present great
diversity both in general character and in detail. We shall find it
both interesting and instructive to consider some of the many different
modes of reproduction, or generation, observed in different classes
of living beings, previous to entering upon the specific study of
reproduction in man. Before doing thus, however, let us give brief
attention to a theoretical form of generation, which cannot be called
reproduction, known as
Spontaneous Generation.--By this term is meant the supposed formation
of living creatures directly from dead matter without the intervention
of other living organisms. The theory is, in substance, an old one.
The ancients supposed that the frogs and other small reptiles so
abundant in the vicinity of slimy pools and stagnant marshes, were
generated spontaneously from the mud and slime in which they lived.
This theory was, of course, abandoned when the natural history of
reptiles became known.
For several thousand years the belief was still held that maggots found
in decaying meat were produced spontaneously; but it was discovered,
centuries ago, that maggots are not formed if the flesh is protected
from flies, since they are the larvae, or young, of a species of this
insect. A relic of the ancient belief in spontaneous generation is still
found in the supposition that horse-hair snakes, so-called, are really
formed from the hairs of horses. This belief is quite common, but
science long ago exposed its falsity.
When the microscope was discovered it revealed a whole new world of
infinitesimal beings which were at first supposed to be of spontaneous
origin; but careful scientific investigation has shown that even these
mere specks of life are not independent of parentage. M. Pasteur and,
more recently, Prof. Tyndall, with many other distinguished scientists,
have demonstrated this fact beyond all reasonable chance for question.
It is, then, an established law that _every living organism originates
with some previously existing living being or beings_.
It may be queried, If it be true that life is but a manifestation of
the ordinary forces of matter,--which are common to both dead and living
matter,--being dependent upon arrangement, then why may it not be that
dead matter may, through the action of molecular laws, and without the
intervention of any living existence, assume those peculiar forms of
arrangement necessary to constitute life, as supposed by the advocates
of the theory in question? It is true that some who recognize the fact
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