question I propose to consider here is simply the mode by which
organic types are preserved as they exist at present. Every one has
a summary answer to this question in the statement, that all these
short-lived individuals reproduce themselves, and thus maintain their
kinds. But the modes of reproduction are so varied, the changes some
animals undergo during their growth so extraordinary, the phenomena
accompanying these changes so startling, that, in the pursuit of the
subject, a new and independent science--that of Embryology--has grown
up, of the utmost importance in the present state of our knowledge.

The prevalent ideas respecting the reproduction of animals are made
up from the daily observation of those immediately about us in the
barn-yard and the farm. But the phenomena here are comparatively simple,
and easily traced. The moment we extend our observations beyond our
cattle and fowls, and enter upon a wider field of investigation, we are
met by the most startling facts. Not the least baffling of these are
the disproportionate numbers of males and females in certain kinds
of animals, their unequal development, as well as the extraordinary
difference between the sexes among certain species, so that they seem as
distinct from each other as if they belonged to separate groups of the
Animal Kingdom. We have close at hand one of the most striking instances
of disproportionate numbers in the household of the Bee, with its one
fertile female charged with the perpetuation of the whole community,
while her innumerable sterile sisterhood, amid a few hundred drones,
work for its support in other ways. Another most interesting chapter
connected with the maintenance of animals is found in the various ways
and different degrees of care with which they provide for their progeny:
some having fulfilled their whole duty toward their offspring when they
have given them birth; others seeking hiding-places for the eggs they
have laid, and watching with a certain care over their development;
others feeding their young till they can provide for themselves, and
building nests, or burrowing holes in the ground, or constructing earth
mounds for their shelter.

But, whatever be the difference in the outward appearance or the habits
of animals, one thing is common to them all without exception: at some
period of their lives they produce eggs, which, being fertilized, give
rise to beings of the same kind as the parent. This mode of genera