As all the organic beings, extinct and recent, which have ever lived on this earth, have to be classed together, and as all have been connected by fine gradations, the best, or, if our collections were nearly perfect, the only possible arrangement, would be genealogical; descent being on this view the hidden bond of connexion which naturalists have been seeking under the term of the natural system. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. In two or more groups of animals, however much they may at present differ from each other in structure and habits, if they pass through closely similar embryonic stages, we may feel almost assured that they have descended from the same parentform, and are therefore closely related. Thus, community in embryonic structure reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups all the developmental stages may have been suppressed, or may have been so greatly modified as no longer to be recognised, through adaptations, during the earlier periods of growth, to new habits of life. Community of descent will, however, often be revealed, although the structure of the adult may have been greatly modified and thus obscured; we have seen, for instance, that cirripedes, though externally so like shellfish, can at once be recognised by their larvæ as belonging to the great class of crustaceans. As the embryonic state of each species and group of species shows us more or less completely the structure of their less modified ancient progenitors, we can see why ancient and extinct forms of life should resemble the embryos of our existing species, their descendants. Agassiz believes this to be a law of nature; but I am bound to confess

that I only hope to see the law hereafter proved true. It can be proved true only in those cases in which the ancient state, now supposed to be represented in existing embryos, has been obliterated neither by the successive variations having supervened at a very early period of growth, nor by the variations having been inherited at an earlier period than that at which they first appeared. It should also be borne in mind, that the supposed law of resemblance of ancient forms of life to the embryonic stages of recent forms, may be true, but yet, owing to the geological record not extending far enough back in time, may remain for a long period, or for ever, incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are second in importance to none in natural history, are explained on the principle of slight modifications not having appeared, in the many descendants from some one ancient progenitor, at a very early period in the life of each, though perhaps caused at the earliest, and having been inherited at a corresponding not early period. Embryology rises greatly in interest, when we thus look at the embryo of an animal as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class.

Rudimentary, Atrophied, and Aborted Organs.

Organs or parts in this strange condition, bearing the stamp of inutility, are extremely common throughout nature. For instance, rudimentary mammæ are very general with male mammals: I presume that the "bastard-wing" in birds may be safely considered as a digit in a rudimentary state: in very many snakes one lobe of the lungs is rudimentary; in other snakes there are rudiments of the pelvis and hind limbs. Some of the

cases of rudimentary organs are extremely curious; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; and the presence of teeth, which never cut through the gums, in the upper jaws of our unborn calves. It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds. Nothing can be plainer than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together!

The meaning of rudimentary organs is often quite unmistakeable: for instance, there are beetles of the same genus (and even of the same species) resembling each other most closely in all respects, one of which will have full-sized wings, and another mere rudiments of membrane; and here it is impossible to doubt, that the rudiments represent wings. Rudimentary organs sometimes retain their potentiality, and are merely not developed this seems to be the case with the mammæ of male mammals, for many instances are on record of these organs having become well developed in full-grown males, and having secreted milk. So again there are normally four developed and two rudimentary teats in the udders of the genus Bos; but in our domestic cows the two sometimes become developed and give milk. In plants of the same species the petals sometimes occur as mere rudiments, and sometimes in a welldeveloped state. In some plants with their sexes separated, the male flowers include a rudiment of a pistil; and Kölreuter found that by crossing a species of this kind with another hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size; and this clearly shows how essen

tially alike in nature the rudiment and the perfect pistil are.

An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other. Thus in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules protected in the ovarium at its base. The pistil consists of a stigma supported on its style; but in some compositæ, the male florets, which of course cannot be fecundated, have a pistil, which is in a rudimentary state, for it is not crowned with a stigma; but the style remains well developed, and is clothed with hairs as in other compositæ, for the purpose of brushing the pollen out of the surrounding and conjoined anthers. Again, an organ may become rudimentary for its proper purpose, and be used for a distinct object: in certain fish the swimbladder seems to be nearly rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung. Other similar instances could be given.

Organs, however little developed, if of use, should not be called rudimentary: they may be called nascent, and may hereafter be developed by natural selection to any further extent. Rudimentary organs, on the other hand, are essentially useless, as teeth which never cut through the gums. As they would be of still less use, when in a still less developed condition, they cannot under the present state of things have been formed by natural selection, which acts solely by the preservation of useful modifications. They relate to a former condition of their possessor, and have been retained, as we shall see, by inheritance. It is difficult to know what organs are nascent; looking to the future, we cannot of course tell how any part will be developed, and whether it is now

in a nascent condition; looking to the past, creatures with an organ in a nascent condition will generally have been supplanted by their successors with the same organ in a more perfect and developed condition, and consequently will not now exist. The wing of the penguin is of high service, acting as a fin; it may, therefore, represent the nascent state of the wing; not that I believe this to be the case; it is more probably a reduced organ, modified for a new function: the wing of the Apteryx, on the other hand, is quite useless, and is truly rudimentary. The simple filamentary limbs of the Lepidosiren apparently are in a nascent state; for, as Owen has recently remarked, they are the "beginnings of organs which attain full functional development in higher vertebrates." The mammary glands of the Ornithorhynchus may, probably, be considered, in comparison with those of the cow, as in a nascent condition. The ovigerous frena of certain cirripedes, which are only slightly developed and which have ceased to give attachment to the ova, are nascent branchiæ.

Rudimentary organs in the individuals of the same species are very liable to vary in degree of development and in other respects. Moreover, in closely allied species, the degree to which the same organ has been rendered rudimentary occasionally differs much. This latter fact is well exemplified in the state of the wings of female moths in certain groups. Rudimentary organs may be utterly aborted; and this implies, that we find in an animal or plant no trace of an organ, which analogy would lead us to expect to find, and which is occasionally found in monstrous individuals of the species. Thus in some Scrophulariaceae we rarely find even a rudiment of a fifth stamen; but this may sometimes be seen plainly or fully developed. In tracing the homologies of the same part in different mem