With this aim they have tried to adapt the phenomena of reproduction among animals to the problem of creation, and to make the beginning of life in the individual solve that great mystery of the beginning of life in the world.  In other words, they have endeavored to show that the fact of successive generations is analogous to that of successive creations, and that the processes by which animals, once created, are maintained unchanged during the period to which they belong will account also for their primitive existence.

I wish, at the outset, to forestall any such misapplication of the facts I am about to state, and to impress upon my readers the difference between these two subjects of inquiry,—­since it by no means follows, that, because individuals are endowed with the power of reproducing and perpetuating their kind, they are in any sense self-originating.  Still less probable does this appear, when we consider, that, since man has existed upon the earth, no appreciable change has taken place in the animal or vegetable world; and so far as our knowledge goes, this would seem to be equally true of all the periods preceding ours, each one maintaining unbroken to its close the organic character impressed upon it at the beginning.

The 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