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 generation is universal, and is based upon that harmonious antagonism between the sexes, that contrast between the male and the female element, that at once divides and unites the whole Animal Kingdom.  And although this exchange of influence is not kept up by an equality of numeric relations,—­since not only are the sexes very unequally divided in some kinds of animals, but the male and female elements are even combined in certain types, so that the individuals are uniformly hermaphrodites,—­yet I firmly believe that this numerical distribution, however unequal it may seem to us, is not without its ordained accuracy and balance.  He who has assigned its place to every leaf in the thickest forest, according to an arithmetical law which prescribes to each its allotted share of room on the branch where it grows, will not have distributed animal life with less care.

But although reproduction by eggs is common to all animals, it is only one among several modes of multiplication.  We have seen that certain animals, besides the ordinary process of generation, also increase their number naturally and constantly by self-division, so that out of one individual many individuals may arise by a natural breaking up of the whole body into distinct surviving parts.  This process of normal self-division may take place at all periods of life:  it may form an early phase of metamorphosis, as in the Hydroid of our common Aurelia, described in the last article; or it may even take place before the young is formed in the egg.  In such a case, the egg itself divides into a number of portions:  two, four, eight, or even twelve and sixteen individuals being normally developed from every egg, in consequence of this singular process of segmentation of the yolk,—­which takes place, indeed, in all eggs, but in those which produce but one individual is only a stage in the natural growth of the yolk during its transformation into a young embryo.  As the facts here alluded to are not very familiar even to professional naturalists, I may be permitted to describe them more in detail.