ing of our land ought to be regulated. In this dispute, however, you will not expect me to take part, for it would be obviously improper. But I may be permitted to remark, that whilst some points of difference between them still remain open for further investigation, a much nearer correspondence of opinion exists with respect to others, than the public in general, or even perhaps the disputants themselves, are inclined to allow. In so far, indeed, as relates to the relative advantages of mineral and ammoniacal manures, I presume there is little room for controversy; for although most soils may contain a sufficiency of the inorganic constituents required by the crop, it by no means follows that the latter are always in an available condition; and hence it may well happen that in most cases in which land has been long under cultivation, the former class of manures becomes, as Baron Liebig asserts, a matter of paramount necessity. Now that the same necessity exists for the addition of ammoniacal manures can hardly be contended, when we reflect, that at the first commencement of vegetable life, every existing species of plant must have obtained its nourishment solely from the gaseous constituents of the atmosphere, and from the mineral contents of the rock in which it vegetated. The only divergence of opinion, therefore, that can arise, relates to the degree of their respective utility in the existing state of our agriculture, and to the soundness of Baron Liebig's position, that a plant rooted in a soil well charged with all the requisite mineral ingredients, and in all other respects in a condition calculated to allow of healthy vegetation, may sooner or later be able to draw from the atmosphere whatever else is required for its full development. And does not, I would ask, this latter position derive some support from the luxuriant vegetation of the tropics, where art certainly contributes nothing towards the result? and is it not also favored by such experiments as those carried on at Lois Weedon in Northamptonshire, where the most luxuriant wheat crops have been obtained for a number of consecutive years without manure of any kind, simply by following out the Tullian system of stirring up and pulverizing the soil? How, too, are we to explain that capacity of subsisting without any artificial supply of ammonia, which Mr. Lawes is led by his experiments to attribute to turnips, and other plants of similar organization, unless we assume that the power residing in the leaves of absorbing ammonia from the air may render plants, in some cases at least, independent of any extraneous aid? Be this, however, as it may, there is at least a wide distinction between this opinion and the one attributed to Baron Liebig by many, who would seem to imagine, that according to his views, ammonia, if derived from artificial sources, was in a manner useless to vegetation. As if it could be a matter of any moment, whether the substance which in both cases afforded the supply of nitrogen, and which in both cases also was primarily derived from the decomposition of organic substances, had been assimilated by plants directly upon its being thus generated, or had been received into their system at a later period, after having been diffused through the atmosphere? To suppose that Baron Liebig should have attached any moment to this distinction seems inconsistent with many passages in his work, in which, although the paramount importance of mineral manures may be insisted upon, and the success which had in certain cases attended the use of one compounded only

of mineral ingredients may be put forward as a motive for further trials, the utility of ammoniacal substances in all their several forms is at the same time distinctly admitted. Still the practical question remains, whether, admitting the theoretical truth of Baron Liebig's position, a larger expenditure of capital will not be required for bringing a given farm into a condition to dispense with ammoniacal manures, than for procuring those materials which contain that ingredient ready for use. And here experimental researches, such as those conducted on so extended and liberal a scale by Mr. Lawes and Dr. Gilbert, come in aid of theory. They stand, as it were, midway between the abstract principles which science points out to the farmer, and the traditional usages with respect to his art which have been handed down to him from one generation to another. They bear the same relation to the farmer, which the records of the clinical practice in a large infirmary do to the general principles of medicine expounded by the modern physiologist. It is true, that the experience of a particular hospital may not at all times coincide with the anticipations which science holds out; but this discrepancy only suggests to us the imperfections of our present knowledge, and it is not allowed to disturb the confidence of the physician in principles already established on incontrovertible evidence. On the contrary, whilst he modifies his practice from time to time by the experience he has gained by actual observation, he feels at the same time the fullest conviction, that these results will be found eventually reconcilable with the general principles which a still more extended series of induction may have established. I need not occupy your time by applying the same method of proceeding to the recent researches alluded to, but I will carry the analogy between the science of Agriculture and Therapeutics one step further. You may recollect, that in a report on the progress of husbandry, drawn up some years ago by one of the most enlightened and zealous promoters of the agricultural interest in Great Britain, it was asserted that chemistry had done nothing for the farmer, except in teaching him to use sulphuric acid with his bones, and to take advantage of the refuse flux liquor formerly thrown away and wasted. Now a statement of this kind, although it might be literally true in the narrow sense in which the author doubtless intended it,—namely, as referring merely to the introduction of new specifics or recipes into farming-was calculated, when put forth on such high authority, to foster that tendency in the human mind to which we are all more or less prone, that of sparing ourselves the trouble of thought and reflection in shaping the course of our conduct, by leaning blindly upon certain rigid and unvarying rules already chalked out to us by others. Grant that science has as yet supplied us with only two infallible recipes for the improvement of our land, the agricultural chemist may derive credit from the reflection, that medicine too, since the days of Hippocrates, has lighted only upon two or three specifics for the cure of disease; and that the most enlightened physicians of the present day, in the spirit which we would fain see actuating the leaders of the agricultural body, depend not upon the efficacy of nostrums, but upon their sagacity in referring the varying conditions of each case which comes before them to those principles of physiology which modern science has established. And has not science also unfolded principles which may be called in to aid and

direct the practical labors of the agriculturist? I need not go further than the works of Baron Liebig for an answer to this question. I may appeal, for instance, to the extensive employment of guano at the present time, first introduced in consequence of his suggestions; I may refer to the substitution of mineral phosphates for bones, founded upon his explanation of the sources from which the latter substance derives its efficacy as a manure; and I may allude more especially to his refutation of the humus theory, to which even the great Saussure gave his adhesion, and the reception of which was calculated to vitiate, not a few processes only, but the entire system of our husbandry.

But it is time to hasten on to certain other departments of Natural Science.

DISCOVERIES IN BOTANICAL SCIENCE.

In Botany and Vegetable Physiology it cannot perhaps be said, that whole provinces have been added to the domain of the science within twenty years, as we have seen to be the case in our review of the progress of chemistry. The improvements in the microscope which have since taken place render us familiar with particulars relating to the structure and functions of the vegetable creation, which the ruder methods of investigation before resorted to would never have revealed to us. We owe to them the interesting discoveries of Brown and Adolphe Brongniart, as to the mode in which the pollen is brought into immediate contact with the ovules, by means of the tubes which it protrudes by a prolongation of the innermost of its two investing membranes. Thus much, at least, appears to be fully ascertained; but, in alluding to the observations of others, who have endeavored to push their scrutiny still further, it becomes me to speak with more diffidence, inasmuch as the office which the pollen discharges in the act of fecundation is still a matter of dispute between such men as Schleiden and Schacht on the one side, and Hofmeister, Moll, &c., on the other. Whilst, however, this controversy continues, it is something at least to know that the vivifying principle, whatever it may be, is actually transmitted to the part where its influence is to be exerted, and not kept apart from it, as we were formerly compelled to assume, by that long intervening plexus of fibres, or tubes, which constitutes the style. To the microscope also we owe all that is as yet known with respect to the reproductive process in cryptogamous plants, which are now shown to possess a structure analogous to that of flowering ones in respect to their organs of reproduction; not, indeed, as Hedwig supposed, that parts corresponding to stamens and pistils in appearance and structure can be discovered in them, but that as the primary distinction of sexes seems to run throughout the Vegetable Kingdom, new parts are superadded to a structure common to all as we ascend in the scale of creation, until from the simple cell, which, in consequence of some differences of structure, to our eyes inappreciable, appears to exercise in one case the function of the male, in another of the female, as is found the case in certain of the Confervæ, we arrive at length at the complicated machinery exhibited in flowering plants, in which the cell containing the fecundating principle is first matured in the stamen, and afterwards transmitted, through an elaborate apparatus, to the

cells of the ovule, which is in like manner enveloped in its matrix, and protected by the series of investing membranes which constitutes the seed-vessel. Thus, as Goethe long ago observed, and as modern physiologists have since shown to be the case, the more imperfect a being is, the more its individual parts resemble each other-the progress of development, both in the Animal and Vegetable Kingdom, always proceeding from the like to the unlike, from the general to the particular. But, whilst the researches of Brown and others have shown that there is no abrupt line of division in the Vegetable Kingdom, and that one common structure pervades the whole, the later inquiries of Suminski, Hofmeister, Unger, Griffith, and Henfrey, have pointed out several curious and unlooked-for analogies between plants and animals. I may mention, in the first place, as an instance of this analogy between plants and animals, the existence of moving molecules, or phytosperms, in the antheridia of ferns and other Cryptogams, borne out, as it has been in so remarkable a manner, by the almost simultaneous observations of Bischoff and Meissner on the egg, confirmatory of those formerly announced by Barry and Newport, and by the researches of Suminski, Thuret, and Pringsheim, with respect to the ovule of plants. I may refer you also to a paper read at the last Meeting of the Association, by Dr. Cohn, of Breslau, who adduced instances of a distinction of sexes which had come under his observation in the lower Algæ. In like manner a curious correspondence has been traced between the lower tribes of animals and plants, in the circumstance of both being subject to the law of what is called alternate generation. This consists in a sort of cycle of changes from one kind of being to another, which was first detected in some of the lower tribes of animals; a pair of insects, for example, producing a progeny differing from themselves in outward appearance and internal structure, and these reproducing their kind without any renewed sexual union,-the progeny in these cases consisting of females only. At length, after a succession of such generations, the offspring reverts to its primæval type, and pairs of male and female insects, of the original form, are reproduced, which complete the cycle, by giving rise in their turn to a breed presenting the same characters as those which belong to their own progenitors. An ingenious comparison had been instituted by Owen and others between this alternation of generations in the animal, and the alternate production of leaves and blossoms in the plant; but the researches to which I especially allude have rendered this no longer a matter of mere speculation or inference, inasmuch as they have shown the same thing to occur in ferns, in lycopodia, in mosses, nay, even in the confervæ. We are indebted to Prof. Henfrey for a valuable contribution on these subjects, given in the form of a Report on the Higher Cryptogamous Plants; from which it at least appears that the proofs of sexuality in the Cryptogamia rank in the same scale, as to completeness, as those regarding flowering plants did before the access of the pollen tubes to the ovule had been demonstrated. Indeed, if the observations of Pringsheim with respect to certain of the Algæ are to be relied upon, the analogy between the reproductive process in plants and animals is even more clearly made out in these lower tribes than it is in those of higher organization. It also appears that the production in ferns and other Acrogens

of what has been called a pro-embryo; the evolution of antheridia and archegonia, or of male and female organs, from the former; and the generation from the archegonia of a frond bearing spores upon its under surface, is analogous to what takes place in flowering plants in general; where the seed, when it germinates, produces stem, roots, and leaves; the stem for many generations gives rise to nothing but shoots like itself: until at length a flower springs from it, which contains within itself for the most part the organs of both sexes united, and, therefore, occasions the reproduction of the same seed with which the chain of phenomena commenced. This is the principle which a learned Professor at Berlin has rather obscurely shadowed out in his treatise on the Rejuvenescence of Plants, and which may perhaps be regarded as one, at least, of the means by which Nature provides for the stability of the forms of organic life she has created, by imparting to each plant a tendency to revert to the primeval type.

DISTRIBUTION OF PLANTS.

To the elder De Candolle we are also indebted for some of our most philosophical views with respect to the laws which regulate the distribution of plants over the globe,-views which have been developed and extended, but by no means subverted, by the investigations of subsequent writers; amongst whom Sir Charles Lyell, in his "Principles of Geology," and the younger De Candolle, a worthy inheritor of his father's reputation, in his recently published work on Botanical Geography, have especially signalized themselves. But it is to the late Prof. Edward Forbes, and to Dr. Joseph Hooker, that we have principally to attribute the removal of those anomalies, which threw a certain degree of doubt upon the principles laid down by De Candolle in 1820, in his celebrated article on the Geography of Plants, contained in the "Dictionnaire des Sciences Naturelles," where the derivation of each species from an individual, or a pair of individuals, created in one particular locality, was made the starting-point of all our inquiries. These anomalies were of two different kinds, and pointed in two opposite directions; for we had in some cases to explain the occurrence of a peculiar Flora in islands cut off from the rest of the world, except through the medium of a wide intervening ocean; and in other cases to reconcile the fact of the same or of allied species being diffused over vast areas, the several portions of which are at the present time separated from each other in such a manner, as to prevent the possibility of the migration of plants from one to the other. Indeed, after making due allowances for those curious contrivances by which Nature has in many instances provided for the transmission of species over different parts of the same continent, and even across the ocean, and which are so well pointed out in De Candolle's original essay, we are compelled to admit the apparent inefficiency of existing causes to account for the distribution of the larger number of species; and must confess that the explanation fails us often where it is most needed, for the Compositæ, in spite of those feathery appendages they possess, which are so favorable to the wide dissemination of their seeds, might be inferred, by their general absence from the fossil Flora, to have diffused themselves in a less degree than many other families