Dr. Hooker's remarks on the limits of species, their dispersion and variation, are striking and instructive. He is of opinion that species vary more, and are more widely diffused, than is usually supposed. Hence he conceives that the number of species has been needlessly and erroneously multiplied, by distinguishing the specimens which occur in different places, and vary in unessential features. He says that though, according to the lowest estimate of compilers, 100,000 is the commonly received number of known plants, he thinks that half that number is much nearer the truth. "This," he says, "may be well conceived, when it is notorious that nineteen species have been made of the Common Potatoe, and many more of Solanum nigrum alone. Pteris aquilina has given rise to numerous book species; Vernonia cinerea of India to fifteen at least. . . . . . . Many more plants are common to most countries than is supposed; I have found 60 New Zealand flowering plants and 9 Ferns to be European ones, besides inhabiting numerous intermediate countries. .... So long ago as 1814, Mr. Brown drew attention to the importance of such considerations, and gave a list of 150 European plants common to Australia." As an example of the extent to which unessential differences may go, he says (p. xvii.,) "The few remaining native Cedars of Lebanon may be abnormal states of the tree which was once spread over the whole of the Lebanon; for there are now growing in England varieties of it which have no existence in a wild state. Some of them closely resemble the Cedars of Atlas and of the Himalayas (Deodar ;) and the absence of any valid botanical differences tends to prove that all, though generally supposed to be different species, are one." Still the great majority of the species of plants in those Southern regions are peculiar. "There are upwards of 100 genera, subgenera, or other well marked groups of plants, entirely or nearly confined to New Zealand, Australia, and extra-tropical South America. They are represented by one or more species in two or more of those countries, and thus effect a botanical relationship or affinity between them all which every botanist appreciates." In reference to the History of Botany, I have received corrections. and remarks from Dr. Hooker, with which I am allowed to enrich my pages. "P. 359. Note. Nelumbium speciosum, the Lotus of India. The Nelumbium does not float, but raises both leaf and flower several feet above the water: the Nymphæa Lotus has floating leaves. Both enter largely into the symbolism of the Hindoos, and are often confounded. "P. 362. Note. For Arachnis read Arachis. The Arachidna of Theophrastus cannot, however, be the Arachis or ground-nut. "Pp. 388 and 394. For Harlecamp read Hartecamp. "P. 394. For Kerlen read Kalm. "P. 394. For Asbech read Osbeck. "P. 386. John Ray. Ray was further the author of the present Natural System in its most comprehensive sense. He first divided plants into Flowerless and Flowering; and the latter into Monocotyledonous and Dicotyledonous :-Floriferas dividemus in DICOTYLEDONES, quarum semina sata binis foliis, seminalibus dictis, quæ cotyledonorum usum præstant, e terra exeunt, vel in binos saltem lobos dividuntur, quamvis eos supra terram foliorum specie non efferant; et MONOCOTYLEDONES, quæ nec folia bina seminalia efferunt nec lobos binos condunt. Hæc divisio ad arbores etiam extendi potest; siquidem Palmæ et congeneres hoc respectu eodem modo a reliquis arboribus differunt quo Monocotyledones a reliquis herbis.' "P. 408. Endogenous and Exogenous Growth. The exact course of the wood fibres which traverse the stems of both Monocotyledonous and Dicotyledonous plants has been only lately discovered. In the Monocotyledons, those fibres are collected in bundles, which follow a very peculiar course :-from the base of each leaf they may be followed downwards and inwards, towards the axis of the trunk, when they form an arch with the convexity to the centre; and curving outwards again reach the circumference, where they are lost amongst the previously deposited fibres. The intrusion of the bases of these bundles amongst those already deposited, causes the circumference of the stem to be harder than the centre; and as all these arcs have a short course (their chords being nearly equal), the trunk does not increase in girth, and grows at the apex only. The wood-bundles are here definite. In the Dicotyledonous trunks, the layers of wood run in parallel courses from the base to the top of the trunk, each externally to that last formed, and the trunk increases both in height and girth; the wood-bundles are here indefinite. "With regard to the Cotyledons, though it is often difficult to distinguish a Monocotyledonous Embryo from a Dicotyledonous, they may always be discriminated when germinating. The Cotyledons, when two or more, and primordial leaves (when no Cotyledons are visible) of a Monocotyledon, are alternate; those of a Dicotyledon are opposite. "A further physiological distinction between Monocotyledons and Dicotyledons is observed in germination, when the Dicotyledonous radicle elongates and forms the root of the young plant; the Monocotyledonous radicle does not elongate, but pushes out rootlets from itself at once. Hence the not very good terms, exorhizal for Dicotyledonous, and endorhizal for Monocotyledonous. "The highest physiological generalization in the vegetable kingdom is between Phanogama and Cryptogama. In the former, fertilization is effected by a pollen-tube touching the nucleus of an ovule; in Cryptogams, the same process is effected by the contact of a spermcell, usually ciliated (antherozoid), upon another kind of cell called a germ-cell. In Phænogams, further, the organs of fructification are all modified leaves; those of Cryptogams are not homologous." (J. D. H.) ZOOLOGY. I have exemplified the considerations which govern zoological classification by quoting the reflexions which Cuvier gives us, as having led him to his own classification of Fishes. Since the varieties of Quadrupeds, or Mammals (omitting whales, &c.), are more familiar to the common reader than those of Fishes, I may notice some of the steps in their classification; the more so as some curious questions have recently arisen thereupon. Linnæus first divides Mammals into two groups, as they have Claws, or Hoofs (unguiculata, ungulata.) But he then again divides them into six orders (omitting whales, &c.), according to their number of incisor, laniary, and molar teeth; namely: Primates. (Man, Monkey, &c.) Bruta. (Rhinoceros, Elephant, &c.) Glires. (Mouse, Squirrel, Hare, &c.) Pecora. (Camel, Giraffe, Stag, Goat, Sheep, Ox, &c.) Bellua. (Horse, Hippopotamus, Tapir, Sow, &c.) In the place of these, Cuvier, as I have stated in the Philosophy (On the Language of Sciences, Aphorism xvi.), introduced the following orders: Bimanes, Quadrumanes, Carnassiers, Rongeurs, Edentés, Pachyderms, Ruminans. Of these, the Carnassiers correspond to the Fera of Linnæus; the Rongeurs to his Glires; the Edentés are a new order, taking the Sloths, Ant-eaters, &c., from the Bruta of Linnæus, the Megatherium from extinct animals, and the Ornithorhynchus, &c., from the new animals of Australia; the Ruminans agree with the Pecora; the Pachyderms include some of the Bruta and the Belluæ, comprehending also extinct animals, as Anoplotherium and Palæothe rium. But the two orders of Hoofed Animals, the Pachyderms and the Ruminants, form a group which is held by Mr. Owen to admit of a better separation, on the ground of a character already pointed out by Cuvier; namely, as to whether they are two-toed or three-toed. According to this view, the Horse is connected with the Tapir, the Palæotherium, and the Rhinoceros, not only by his teeth, but by his feet, for he has really three digits. And Cuvier notices that in the two-toed or even-toed Pachyderms, the astragalus bone has its face divided into two equal parts by a ridge; while in the uneven-toed pachyderms it has a narrow cuboid face. Mr. Owen has adopted this division of Pachyderms and Ruminants, giving the names artiodactyla and perissodactyla to the two groups; the former including the Ox, Hog, Peccary, Hippopotamus, &c.; the latter comprehending the Horse, Tapir, Rhinoceros, Hyrax, &c. And thus the Ruminants take their place as a subordinate group of the great natural even-toed Division of the Hoofed Section of Mammals; and the Horse is widely separated from them, inasmuch as he belongs to the odd-toed division. As we have seen, these modern classifications are so constructed as to include extinct as well as living species of animals; and indeed the species which have been discovered in a fossil state have tended to fill up the gaps in the series of zoological forms which had marred the systems of modern zoologists. This has been the case with the division of which we are speaking. Mr. Owen had established two genera of extinct Herbivorous Animals, on the strength of fossil remains brought from South America: -Toxodon, and Nesodon. In a recent communication to the Royal Society he has considered the bearing of these genera upon the divisions of odd-toed and even-toed animals. He had already been led to the opinion that the three sections, Proboscidea, Perissodactyla, and Artiodactyla, formed a natural division of Ungulata; and he is now led to think that this division implies another group, 66 a distinct division of the Ungulata, of equal value, if not with the Perissodactyla and Artiodactyla, at least with the Proboscidea." This group he proposes to call Toxodonta. 2 Owen, Odontography. Phil. Trans., 1853. BOOK XVII. PHYSIOLOGY AND COMPARATIVE ANATOMY. VEGETABLE MORPHOLOGY. I Morphology in Linnæus. HAVE stated that Linnæus had some views on this subject. Dr. Hooker conceives these views to be more complete and correct than is generally allowed, though unhappily clothed in metaphorical language and mixed with speculative matter. By his permission I insert some remarks which I have received from him. The fundamental passage on this subject is in the Systema Natura; in the Introduction to which work the following passage occurs: "Prolepsis (Anticipation) exhibits the mystery of the metamorphosis of plants, by which the herb, which is the larva or imperfect condition, is changed into the declared fructification: for the plant is capable of producing either a leafy herb or a fructification. . . . "When a tree produces a flower, nature anticipates the produce of five years where these come out all at once; forming of the bud-leaves of the next year bracts; of those of the following year, the calyx; of the following, the corolla; of the next, the stamina; of the subsequent, the pistils, filled with the granulated marrow of the seed, the terminus of the life of a vegetable." Dr. Hooker says, "I derive my idea of his having a better knowledge of the subject than most Botanists admit, not only from the Prolepsis, but from his paper called Reformatio Botanices (Amæn. Acad. vol. vi.); a remarkable work, in respect of his candor in speaking of his predecessors' labors, and the sagacity he shows in indicating researches to be undertaken or completed. Amongst the latter is, V. 'Prolepsis plantarum, ulterius extendenda per earum metamorphoses.' The last word occurs rarely in his Prolepsis; but when it does it seems to me that he uses it as indicating a normal change and not an accidental one. |