branches of the tree be broken off by the high winds of winter, and when spring comes they will attract the sap which went to those branches to themselves. This will arouse their dormant energies so powerfully that they will force their way through the wood and bark to the surface, though that wood may be the growth of years. The bud which has slept in a condition of feebleness, perhaps for half a century, will break forth at last into a powerful branch, the injury done by the hostile forces of nature will be repaired, and the tree will carry on successfully the battle for life.

All must be familiar with the sight of willows and other trees whose main branches have been thus broken off, and whose trunks are nevertheless covered with young branches and shoots, the growth of buds which have been buried in the wood and for years dormant beneath their surface.

We have thus, we hope, placed before the reader a simple and accurate method of studying the growth of trees. Of the tree it may be truly said that THE WHOLE IS REPRESENTED IN

EACH OF ITS PARTS.

EXPLANATION OF PLATE XVIII.

Branch of Common Beech (Fagus sylvatica), showing its annual growths for thirteen years. The sets of annuli show the successive points where the growth was stopped during winter, and the figures opposite, the year when this occurrence took place.

REVIEWS.

THE

COMPARATIVE

PHYSIOLOGY.*

HE accomplished author of the Souvenirs d'un Naturaliste has produced under this title a very readable little book, which treats of one of the most complicated questions in physiology-that, namely, which has reference to the extraordinary variety of forms derivable from the same species among some of the lower animals, their mutual relation, and the analogies which may be discovered between them and some of the phenomena exhibited in the reproductive processes of the higher animals. There are few questions in the range of physiological inquiry which have raised greater feelings of wonder, or which have given rise to more speculation than those which have received the name of "alternate generation," "parthenogenesis," &c., while at the same time the obscurity and complexity of many of the phenomena have stimulated the curiosity of some, and deterred other investigators from entering upon a subject of so much difficulty.

M. de Quatrefages commences his work by pointing out that change is inherent in animal bodies, the highest no less than the lowest-plants and animals are momentarily losing some of their substance, which must be renewed; and this applies to all parts alike, the most delicate, and those which are most assimilated to inorganic bodies. He points out the defects in the general acceptation of the term metamorphosis, and proposes three expressions, each having a distinct meaning, to be applied to the very different forms of change which take place in various grades of the organic kingdom, viz., transformation, which includes the whole of the changes which a germ undergoes in its progress to the condition of an embryo; metamorphosis he retains to imply the changes which the animal undergoes after exclusion, and which profoundly alter the general forms, or the modes of life of the individual; while he proposes the new term geneagenesis for those changes which refer to the generations themselves a term derived from yɛvɛa, birth, and yɛvɛous, generation.

The transformations, then, which take place in the egg, or ovum, next occupy the attention, from the first appearance of organized tissue through all those remarkable stages which have of late years been so carefully and elaborately worked out and watched, more particularly by

"The Metamorphoses of Man and of Animals," by A. DE QUATREFAGES. (Métamorphoses de l'Homme et des Animaux, par A. DE QUATREFAGES.) Paris: J. B. Baillière, 1862.

German physiologists. The author, in passing, pays a just tribute to the celebrated Schwann, whose theories, at one time almost universally accepted, have long since been found to be formed upon false premisses and imperfect observations. But, as he observes, like all general doctrines which bind together a great number of isolated facts, this cellular theory has at once cleared and widened the field of research. Transformation, however, in the form of growth, continues to take place in the higher animals after exclusion, but it is worthy of remark, that it is often of a kind which Milne Edwards designates as recurrent development, as in the case of the higher monkeys, which at birth not a little resemble man, but, from a certain moment, their development, instead of raising them, lowers them in the animal scale, making more and more wide the breach between them and the human species.

The metamorphoses of animals in their restricted sense are of course best illustrated by those which are exhibited in the class of insects, in which there is apparently a sudden change from the grub to the chrysalis; and from the chrysalis to the imago or perfect insect. But these changes are really very gradual, and the aphorism "Natura nihil fit per saltum," is fully borne out in their case. We have not space to follow our author through the interesting and lucid details which he gives concerning the metamorphoses of insects in general, of myriapods, crustacea, annelids, molluscs, reptiles, and batrachians, all of which exhibit features of striking and peculiar interest. Instead, however, of regarding the various forms presented by an insect, for example, in each of its metamorphoses, as distinct beings, of which the first inclosed and nourished the second, (as Réaumur would have done,) by the light of modern science we are enabled to perceive that they are but the same being in stages analogous to those presented to our notice by the embryo, the foetus, and the young of mammiferous animals. And the very proof which Réaumur invoked in favour of his view, viz., the dissections of Swammerdam, who could distinguish in the advanced caterpillar the antennæ, wings, &c., of the perfect insect, shows the gradual and progressive nature of the change to which they are subject. The caterpillar for us is an embryo which, in order to become adult, has not only to grow, and to develop itself as an infant would, but also has to pass through certain changes analogous to the embryonic changes of higher animals. The rapidity of growth, too, which is so remarkable a character in the primary conditions of insects, is one which is eminently analogous to the embryonic condition; and thus a larva may be considered as an independently existing embryo, which nourishes itself, instead of being fed by its mother, and undergoes externally, before our eyes, changes or transformations analogous to those which, in the case of viviparous animals, are accomplished within the maternal organism.

The phenomena of the third kind, which are placed under the collective title of geneagenesis, dovetail, as it were, into the other two, as facts of progressive growth and individualization. Here we find animals which, rigorously speaking, appear to have neither father nor mother, but only a parent, which forms them at the expense of its own substance. We find, as it were, sons which never resemble their father, and which produce

children for ever different from themselves. We see a single germ produce not one individual, but multitudes of individuals, and sometimes several generations, which have no relation between them of form, structure, or mode of life. And, lastly, we see the germ lose its primitive individuality, and give place to a crowd of new individualities before the products of this very germ are arrived at the perfect condition.

The discoveries long since made by Peysonnel, Trembley, and others upon the generation of Hydra by means of gemmation and fission, and which caused at the time such universal interest, gave a great impulse to the early development of this question, and established the important fact that certain animals could, like plants, reproduce themselves by buds, appearing at first sight a deathblow to the doctrine of pre-existent germs. The succeeding great discovery, the origin of which is due to Chamisso, who announced in 1819 the alternations of generations of biphora (or salpa), seemed as incredible as the adventures of Peter Schlemihl, his own "shadowless man." The new doctrine was first denied; then, when facts appeared too stubborn, attempts were made to explain them away; but the succeeding investigations of Sars and Von Siebold established incontestably the truth of these extraordinary disclosures of the secret workings of nature.

But the isolated observations of the first explorers of this new field were insufficient to give them any clue to the significance of these wonderful revelations. Modern science, however, has thrown much light upon the problem, and all these modes of generation are found to have something in common. In the case of the hydra, which reproduces itself by eggs, as well as by buds, it is as though there issued from the egg of a butterfly an animal having all the external character of the perfect insect, but deprived of organs of reproduction, which should produce by gemmation or budding beings like itself, and susceptible, as well as itself, of acquiring later the attributes of father and mother. In the case of the compound ascidians, it is as though the egg of the butterfly had produced first a caterpillar, and that this has arrived at its perfect condition; then, on this butterfly issuing from the primitive egg other butterflies like the first have made their way (poussé), of which the original one is, properly speaking, neither the father nor the mother, but only the parent.

Geneagenesis does not exhibit itself among true mollusca; but in the division of the molluscoids it appears to be the general rule. All these animals are more or less near in position to the biphora.

Among the echinodermata there appears to be a sort of connecting link between the two orders of facts above alluded to, and the gulf between the two is thus bridged. The whole animal comes under the category of geneagenesis; but many of the important internal organs undergo a simple metamorphosis. The doctrine of spontaneous generation, which has so often been bolstered up by the phenomena presented by the propagation of intestinal worms, can no longer resort to this support, since it is now found that the remarkable phenomena of alternate generation are exhibited in a strongly marked manner in this division of animals. All the pretended agamic species are now known to be only different phases of development of sexual species. These changes have, in many cases, been accurately followed, and analogy would justify us in believing that

the same obtains with the remainder. We can, with more and more certainty, echo the aphorism of Harvey, “Omne vivum ex ovo." The phenomena of geneagenesis mask, but do not alter, the principle of this great truth. Buds are the products of pre-existing germs,-secondary germs, in fact; and a gemmiparous reproduction suffices not to perpetuate the species, for at the end of a certain time reproduction by an egg becomes absolutely necessary. This alone is a function of the first order, to which gemmation is only subordinate.

In the latter part of his work, M. de Quatrefages combats the opinion of Professor Owen, of whom he speaks with the greatest respect, on the subject of parthenogenesis, and offers his own views of these phenomena; pointing out that, like multiplication by buds, or by suckers, natural or artificial, or by alternate generation, all of which he believes to be manifestations of one and the same grand phenomenon, so also parthenogenesis, or virginal reproduction, is but a particular case of geneagenesis, and, therefore, comes in the same category as those before mentioned.

The work of M. de Quatrefages abounds with original views and clear expositions of complicated and obscure phenomena, which he has brought into an ingenious and useful connection. We look upon it as the best that has appeared upon the difficult subject of which it treats, and should like to see it laid before the public in an English dress.

The Tropical World: a popular Scientific Account of the Natural History of the Animal and Vegetable Kingdoms in the Equatorial Regions. By Dr. G. HARTWIG, Author of "The Sea and its Living Wonders." London: Longman & Co., 1863.

TH

HERE are, perhaps, few things which exercise so fascinating an influence upon the imagination of the student of natural history as the glowing descriptions given by travellers, of the aspects of nature in the tropical regions of the globe. Familiar as we all are with the moderate vegetation, with the sober tints of the birds and insects, and with the generally unimposing and harmless character of the quadrupeds and reptiles of our own temperate climes, the contemplation of the marvels of form, of size, of strength, and of colour, exhibited by the productions of the tropics, inspire us at once with admiration and with awe. Beautiful as are our leafy groves of oak and beech, resonant with the melodious voices of browncoated birds, and tenanted by the stately forms of oxen or deer, they lack at once the grandeur and the grotesqueness of the strange and giant vegetation of the tropical forest; they lack the wondrous birds which, like living gems, dazzle the sight, as they flash through the sunshine, or settle in the shade; they lack the strange and unearthly sounds of the campanero, the howler, and the whip-poor-will, which seem to enhance the mystery of its gloomy recesses. Guided by the vivid descriptions of the veterans Humboldt and Waterton, we can picture to ourselves the pathless and interminable forests of Guiana, where Nature exuberates in perfect freedom from human interference-now revelling in dense and impene