504 ment. EPOCHS OF DIFFERENTIATION. series, such a perfect uniformity in the condition of temperature obtained, the same is often observed in the first periods of individual developThe circumstances under which the ovum commences its career, even in the highest tribes, insure for it a perfect relief from every variation of heat. Included in the body of the female, it is cut off from all external causes of disturbance, and kept at the temperature of her body, whatever that temperature may be. In those cases, as in birds, in which the embryo is developed under circumstances of necessary exposure, a strong instinct is called into operation, and, by the incubation of the parent, the necessary uniformity is secured. Again, in other instances, as in the ova of insects, which, by reason of their minuteness and their frequently exposed position, although they may run through their earlier changes with relatively great rapidity, some accomplishing them in the almost uniform warmth of a summer's day, development never does nor can occur until the required condition, even if it be temporary, as to uniformity of temperature, is reached. These considerations, though not affording an absolute proof that the career of development is guided by the influence of external physical conditions, are sufficiently significant to cast an air of probability over that doctrine; and even if we adopt the view that the developing germ possesses a plastic power, which spontaneously compels it to run forward from stage to stage in a predestined career-if we recall what has already been said respecting that plastic power, that perhaps it is itself nothing more than a manifestation of the remains of antecedent physical impressions, we are really brought back to the same starting-point; and, under any hypothesis, we encounter, sooner or later, as a necessary postulate, the grand doctrine that, directly or indirectly, development is a function of external physical condition. It is not to be supposed that differentiation takes place with equal Epochs of dif- ease at all periods of the history of organic forms, whether ferentiation. we consider them in the great scale, as constituting the animal series, or on the small, in the individual. There are undoubtedly epochs in each of their histories at which the exertion of an external influence will produce an effect infinitely greater than that which would occur at any other moment. If we may be permitted to use such a mechanical illustration, the career of an organism recalls the flight of a heavy projectile, as a shell, thrown upward, which, at the first moments of its ascent and the last of its descent, pursues its way irresistibly, but when it is at the top of its flight, and the momentum which had been imparted to it is just ceasing, the slightest breath of air, or the exertion of any other insignificant force, will divert it into a path different from that in which it would have gone; and so, in the career of an organism, there are moments when forces, which, at another time, would have been unfelt, REPRODUCTION AND DEVELOPMENT. 505 can bring on differentiation, and, through it, call into existence new functions, and thereby forever determine a new course, through which it must pass. It is because a due weight has not been given to this consideration that many physiologists have depreciated the influence of external circumstances, or even denied it altogether, for they have assumed that, since we can not produce a more marked change than we do in the way of accomplishing a variation in species by artificially altering the conditions under which they exist, such conditions can have had but little power in bringing them to their present state. Organic chanes occur chiefly in the first periods of life. Upon the whole, there can be no doubt that differentiation will occur in a more marked manner according as the exciting impression is made at an earlier period of the organic career. Conversely, the more advanced the organism, the less the probability of differentiation. For this reason it is that striking changes of this kind are rarely witnessed in individual life: they occur chiefly in the first embryonic states, and therefore, for the most part, require for their full manifestation generation after generation. Great organic variations are not, then, to be expected in the individual, though they may be distinctly manifested in the course of time by the race to which it belongs. CHAPTER IV. OF REPRODUCTION AND DEVELOPMENT. Relation of Organic Beings: they come from a similar Cell and develop to different Points.-Their Division by Classification is fictitious.--Development and Differentiation.—Homogenesis and Heterogenesis.—They depend on physical Conditions.—The reproductive State closes Development. Development is from the General to the Special-Law of Von Bär.-Invariable Sequence in Differentiation. OF REPRODUCTION: 1st. By Generation.—Conjugation and Filaments.-The Sperm-cell: its Production.-Spermatozoa.-The Germ-cell: its Production. Ovum in the Ovary.—Its Structure.—Corpus Luteum. Ovum in the oviduct.—Mulberry Mass.-Germinal Membrane.—The Chorion. Ovum in the Uterus.-Membrana Decidua.-Placenta.-Development of the Embryo.-Types of Nutrition.-Of Conception.-Of Gestation.—Of Parturition.-Influence of both Parents. 2d. By Gemmation.-Budding of Plants and Animals.—Of Grafting.-Limit of Gemmation.— Influence of Temperature on Gemmation. Alternations of Generation.-Its Explanation. In the popular view of the organic world, each individual being is regarded as maintaining an existence independent and irre- Popular view spective of all others, or, at most, only connected with those of the indeof its own race or kind. Without any apparent disturb- ganic beings. pendence of or 506 THE PRIMORDIAL GERM. ance of the general system, this or that species or genus might never have existed, since it stands in no relation as being the product of others, nor as having been concerned in giving origin to others. Its error. But these superficial conceptions are now to be replaced by others of a far more general and philosophical order, which present to us organic creation under an aspect of sublime grandeur, each class of beings standing in an intercommunication or connection with othersa part of a plan, the manifestations of which are not limited to the forms now existing, but also include those presented by the ancient geological times. These views cast a flood of light not only upon the relations of the various races of life to one another, but also of the human family to them, illustrating the course through which man has hitherto passed, and indicating that through which, in future ages, he is to go. the same germ or cell. Starting from a solitary cell, development takes place, and, according All organic be- as extraneous forces may be brought into action, variable in ings start from their nature, and differing in their intensity, the resulting organisms will differ. If such language may be used, the aim of Nature is to reach a certain ideal model or archetype. As the passage toward this ideal model is more or less perfectly accomplished, form after form, in varied succession, arises. The original substratum or material is in every instance alike; for it matters not what may be the class of animals or of plants, the primordial germ, as far as investigation has gone, is in every instance the same. The microscope shows no difference, but, on the contrary, demonstrates the identity of the first cell, which, if it passes but a little way on its forward course, ends in presenting the obscure cryptogamic plant, or, if it runs forward toward reaching the archetype, ends in the production of man. The diversity of form that is eventually presented depends then, not upon the constitution or aspect of the primitive cell, but upon the influence of the many surrounding agencies to which it is exposed. In one instance, through The primitive the interworking of these agencies-perhaps by cessation of ward to differ one, or perhaps by its increased intensity-development ent points. comes up rapidly to a certain point, and there stops. In another case, through change in the conditions, it runs to a farther degree, and there stops. Organic beings are, therefore, the materialized embodiment of what must take place through the action of given forces, of a given intensity, and under given conditions, on an evolving cell; The classifica- and, though it may suit the purposes of description to classify tions of natural them into orders, genera, species, or other such subdivisions, it must never be forgotten that these are artificial fictions, and have no real foundation in nature. cell passes on history are fic titious. Not only is the primordial cell in all instances the same, but the first stages of its career are in all instances identical, and this whether we VALUE OF EMRRYONIC FORMS. 507 consider it in the lowest or the highest cases, belonging either to the vegetable or the animal kingdom. It is a process of repetition or reproduction, cell arising from cell. And here at once we may correct the language so often used—indeed, which we have ourselves just used in this respect, for such terms as high and low are only to be employed in a very restricted sense. The evolving cell gives rise to other cells, but for a period of time no indication is presented as to which of the two kingdoms it is to belong, animal or plant. By degrees, as the develop- Development ment goes on, that point is determined, and so, one after another, the unfolding mass gradually reveals the class, order, evolving of pefamily, genus, species, and, finally, its sex and individual culiarities. In all this there is an evolving of the special out of the general; one after another, peculiarities, which are more and more minute, arise; and thus we are not to regard the progress of development as taking place from the lower to the higher, forms that are more and more complex arising in succession, but we are to regard it as the gradual unfolding of the special from the general. pe is attended by the gradual culiarities. the individual This career of development applies equally to the case of any individual animal, or any race of animals. Thus man himself, in Analogy of desuccession, passes through a great variety of forms, from the velopment in condition of a simple cell; these forms merging by degrees and in the race. into one another, the form of the serpent, of the fish, of the bird, and this not only as regards the entire system in the aggregate, but also as regards each one of its constituent mechanisms-the nervous system, the circulatory, the digestive. Now, on the passage onward, these forms are to be regarded, as has been well expressed, each one as the scaffolding by which the next is built; and just as man, in his embryonic transit, presents these successive aspects on the small scale, so does the entire animal series present them in the world on the great scale. Races of animals are not to be compared as though they were more perfect or lower than one another, but as having advanced more or less in the direction from the general to the special; and therefore, in this philosophical view, we are justified in regarding those animated forms which heretofore have been spoken of as lower in the animal scale as being, in reality, the embryos of those that are higher; and this should lead us to a juster estimate of their relation of value toward one another, since we are very apt to contrast them in that respect. In the case of an individ- Value of emual, as in man, we put at once a true interpretation on the bryonic forms. value of the various transitory conditions through which he has passed, estimating these as of but little intrinsic importance; as being, as it were, no more than links in a chain; and this may teach us a more just appreciation of the relations of animal races to one another and to the human species. It may teach us the folly of comparing, as some have endeav 508 DEVELOPMENT OF THE SPECIAL FROM THE GENERAL. ored to do, the animal tribes with ourselves; of measuring their instincts with our mental operations; things which are different terms of two different series, and things which are incommensurable. eral. There are three cases in which we might consider this career. These are, first, in the development of particular organs, as the digestive, respiratory, or circulatory; second, in the development of individual beings, which pass in their onward progress, as we have said, through various forms in succession; third, in the development of species, presenting what have been formerly designated as successive stages of increasing perfection. For all these various cases a single illustration may suffice. Illustration of Thus, in the primitive period of life, a single membrane disthe unfolding charges promiscuously and contemporaneously all the vaof the special from the gen- rious organic functions-it digests, it respires, it secretes ; but, a little advance onward, special portions of it are allotted for one and another of these uses, and a localization, a centralization of function ensues, and things that were mixed in confusion become separate and distinct. As the passage onward is made, still farther specializations are introduced, and so on in succession. Thus at the two extremes we may contemplate the single germinal membrane of the ovum, which is discharging contemporaneously every function-digesting, absorbing, respiring, etc.—and the complete organic apparatus of man, the stomach, the lungs, the skin, the kidneys, and the liver-mechanisms set apart each for the discharge of a special duty, yet each having arisen, as we know positively from watching their order of development, from that simple germinal membrane. We must not, therefore, permit ourselves to be deceived by the appearance of complexity they exhibit, since, intricate as may be their construction, they have all arisen through gradual centralization, one duty being separated from another, and having an appropriate mechanism for itself; and so, at last, it comes to pass that even the minutest conditions are discharged by a special part. Thus, in the kidney, the salts are removed by one portion of the structure and the organic constituents by another; yet, even in these utmost conditions of refinement, the primitive condition is at all times ready to be reproduced, and, when driven to it, each of these structures can act vicariously for the others, and discharge for the others their duty. It is unnecessary for our purpose to multiply instances, since every page of natural history, comparative anatomy, and embryology presents them in abundance; but it may be to the purpose to remark that this doctrine leads to more worthy conceptions of the system of nature; for if we suppose that there has been, in the case of the animal series, a passage from things that are less perfect to things that are more so, though this may be agreeable to our own experience, which is essen |