known. Their beautiful shapes and colours have been reproduced in
hundreds of exquisite pictures by our great artist-naturalists. We
thus have to recognise that there are two distinct kinds of
reproduction in living things. One is "asexual," by means of division
or separation of large or special masses of their existence, made up
of ordinary tissue cells. Co-existing with this, often in the same
individuals, is the other method, the "sexual," by means of detached
egg-cells and sperm-cells which are thrown off from the parents, and
do not (except in rare instances) proceed to develop unless the
egg-cell is "fertilised" by the fusion with it of a sperm-cell.

The whole subject of the reproduction of animals and plants was, until
the introduction of the microscope, involved in obscurity and mystery.
The Greeks and Romans had necessarily very imperfect and erroneous
notions on the subject, and it was not until 300 years ago that
William Harvey, the discoverer of the circulation of the blood,
declared, as a general law, that every living thing is born from an
egg. During that 300 years his conclusion has been examined and
modified, corrected and expanded, and the microscope has at last
enabled us to see and follow the excessively minute particles and
structures by which sexual reproduction is effected. Harvey's dictum
was a step in advance when it was made, for previously the belief was
current that living things were "bred" in all sorts of queer ways. It
was supposed that the putrefying flesh of a dead animal actually was
converted by a sudden process into maggots, and that rotten wood
would breed, out of its own substance, ships' barnacles and even young
geese and mice--an opinion contested only 200 years ago by Sir Thomas
Browne! No difficulty was felt in admitting that whole swarms of
insects, fishes, and even herds of larger beasts were spontaneously
generated from mud, from putrid matter, or from the waters of the sea.
That, indeed, was the popular notion set forth by the poet, John
Milton, as to the mode in which living things were "miraculously"
brought into existence at the beginning of things by the "fiat" of the
Creator. What more probable than that such a creation should still be,
here and there, at work? However, not three centuries ago, actual
experiment gradually convinced the learned that maggots are bred in a
dead body only from the eggs laid by parent flies, as shown by the
Italian Redi in 1668 who found th