METHODS OF STUDY IN NATURAL HISTORY.
All important changes in the social and political condition of man, whether brought about by violent convulsions or effected gradually, are at once recognized as eras in the history of humanity. But on the broad high-road of civilization along which men are ever marching, they pass by unnoticed the landmarks of intellectual progress, unless they chance to have some direct bearing on what is called the practical side of life. Such an era marked the early part of our own century; and though at the time a thousand events seemed more full-freighted for the world than the discovery of some old bones at the quarry of Montmartre, and though many a man seemed greater in the estimation of the hour than the professor at the Jardin des Plantes who strove to reconstruct these fragments, yet the story that they told lighted up all the past, and showed its true connection with the present. Cuvier, as one sees him in a retrospective glance at the wonderful period in which he lived, and which brought to the surface all its greatest elements,—one among a throng of exceptional men, generals, soldiers, statesmen, as well as men of commanding intellect in literary and scientific pursuits,—seems always standing at the meeting-point between the past and present. His gaze is ever fixed upon the path along which Creation has moved, and, as he travels back, recovering step by step the road that has been lost to man in apparently impenetrable darkness and mystery, the light brightens and broadens before him, and seems to tempt him on into the dim regions where the great mystery of Creation lies hidden.
Before the year 1800, men had never suspected that their home had been tenanted in past times by a set of beings totally different from those that inhabit it now; still farther was it from their thought to imagine that creation after creation had followed each other in successive ages, every one stamped with a character peculiarly its own. It was Cuvier who, aroused to new labors by the hint he received from the bones unearthed at Montmartre, to which all his vast knowledge of living animals gave him no clue, established by means of most laborious investigations the astounding conclusion, that, prior to the existence of the animals and plants now living, this globe had been the theatre of another set of beings, every trace of whom had vanished from the face of the earth. To his alert and active intellect and powerful imagination a word spoken out of the past was pregnant with meaning; and when he had once convinced himself that he had found a single animal that had no counterpart among living beings, it gave him the key to many mysteries.
It may be doubted whether men's eyes are ever opened to truths which, though new to them, are old to God, till the time has come when they can apprehend their meaning and turn them to good account. It certainly seems, that, when such a revelation has once been made, light pours in upon it from every side; and this is especially true of the case in point. The existence of a past creation once suggested, confirmation was found in a thousand facts overlooked before. The solid crust of the earth gave up its dead, and from the snows of Siberia, from the soil of Italy, from caves of Central Europe, from mines, from the rent sides of mountains and from their highest peaks, from the coral beds of ancient oceans, the varied animals that had possessed the earth ages before man was created spoke to us of the past.
No sooner were these facts established, than the relation between the extinct world and the world of to-day became the subject of extensive researches and comparisons; innumerable theories were started to account for the differences, and to determine the periods and manner of the change. It is not my intention to enter now at any length upon the subject of geological succession, though I hope to return to it hereafter in a series of papers upon that and kindred topics; but I allude to it here, before presenting some views upon the maintenance of organic types as they exist in our own period, for the following reason. Since it has been shown that from the beginning of Creation till the present time the physical history of the world has been divided into a succession of distinct periods, each one accompanied by its characteristic animals and plants, so that our own epoch is only the closing one in the long procession of the ages, naturalists have been constantly striving to find the connecting link between them all, and to prove that each such creation has been a normal and natural growth out of the preceding one. 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 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.