studied the phenomena presented by Plato, is that the darkening of the floor of Plato results from an actual change due to heating action of the solar rays. Either from the volatilisation of some constituent of the substance composing the interior of Plato, or from some change in its constitution due to the heating effect of the solar rays, the floor darkens in colour. Now, there are more than one substance, known as a constituent of the crust of the earth, that thus darkens when submitted to heat,—some from loss of moisture, others from loss of other substances, and others again apparently from a molecular action. Selenographers cannot say which of these probably forms the floor of Plato, or rather to which of these does the substance probably forming the floor of Plato bear most resemblance; but there is every reason to believe that to one of them the floor of Plato bears a strong analogy.

It has been urged by Mr. Proctor (loc. cit., p. 504) that under these conditions the maximum change on Plato would not occur until long after the sun had attained its meridianal altitude on Plato, whereas observation shows that this takes place shortly after this period. It can be shown that this is not necessarily the case, because the time when the effect of the solar rays reaches its maximum power depends very greatly on the action exerted by the lunar atmosphere. There is not merely a heating action only, but a loss of heat through various causes; so that the problem is complex, and involves the balance of action between the opposite effects. The question of the effect of the lunar atmosphere in modifying the action of the solar rays upon the moon, though one of very great interest, is one involving very great difficulties, more especially as observation and theory appear to be at variance in one or two points. The influence of the lunar atmosphere is, however, sufficiently great to remove the difficulty that has been urged on this point, just in the same manner that the action of the terrestrial atmosphere renders the hottest portion of the terrestrial day close after noon, rather than towards evening. The real power of this active agent, the lunar atmosphere, is rarely properly appreciated, and, in fact, the whole question of the atmospheres of the planets, as if it were forgotten that the whole subject is within certain limits as reducible to the control of the present powers of mathematical analysis as the motions of the planets themselves. In a future number this subject may be returned to, and the real power exerted by a lunar atmosphere on the phenomena presented by the moon, and on the condition of its surface, will be considered.

The instances that have been dealt with in the preceding pages will show that selenographers are not without strong evidence in favour of the opinion that has long been unanimously held by them, that processes of change are still actively at work upon the moon. It must not, however, be supposed that the above are the sole instances which they have recognised, because this is by no means true, only the difficulties in establishing others have led to their omission here. Dealing with a subject such as Selenography, it is only those who are familiar with all its details who properly appreciate the evidence in favour of or against its problems, just as in the branches of mathematical astronomy, dealing with our satellite, it is only a proficient in mathematical astronomy who could appreciate the difficulties of the lunar theory, and feel a true confidence in the correctness of the manner in which they have been overcome. The difficulties in the way of making the true bearings of selenographical questions properly understood are greater than might be imagined; for even the very elementary fact that volcanic changes such as are now active on the earth would not be recognisable on the moon, in the present state of our acquaintance with the configuration of its surface, is not generally understood. When, however, the attention of astronomers is more generally directed to the study of the lunar surface, Science will be greatly the gainer, as it is there that the past and future of our earth is to be learnt.

II. EVOLUTION BY EXPANSION,

VERSUS

EVOLUTION BY NATURAL SELECTION.

By J. HUDDART.

HERE is nothing more difficult than to disembarrass the mind of a preconceived idea. A belief instilled in childhood and entertained through life, without a shadow of suspicion as to its truth, takes such firm hold of the imagination as positively to unfit the reason to judge impartially of any opinion opposed to that belief, however false and monstrous it may be.

The tradition of the fixity of species has become so deeply rooted in the human mind that only time, overwhelming evidence, and the untiring efforts of such pioneers of thought as Darwin, will ever eradicate the belief in it. He has already combated this belief with such success that he has considerably shaken the opinions on the subject of many eminent naturalists, while he has made more converts to the doctrine of Descent than is generally supposed. He was by no means the first naturalist to discard the old theory of Creation. He, moreover, disclaims to have been the first to have accounted for the gradual development of species by natural selection, stating that in this he was long preceded by Dr. Wells and Mr. Mathews. Owing, however, to the labour and thought he has expended in elaborating the theory, it will doubtless be always chiefly associated with his name. He considers natural selection to have been the main instrument in the formation of species, but allows that sexual selection, climate, geographical distribution, and other minor influences have had their share in the work.

The striking similarity in appearance, habits, and structure presented by many species of plants and animals first caused naturalists to doubt the "Fixity of Species," and to seek a reason for "Unity of Type" as it is called. According to Darwin, "Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or the doctrine of final causes. The hopelessness of the attempt has been expressly admitted by Owen in his most interesting work on the Nature of Limbs. On the ordinary view of the independent creation of each being, one can only say that so it is;-that it has pleased the Creator to construct all animals and plants on a uniform plan but this is not a scientific explanation."

The most obvious way of accounting for the phenomenon is by concluding that species possessing similar characteristics are related to one another. This is the doctrine of descent. We may go further, and presume that families, genera, and classes presenting points of similitude are also related.

But assuming the truth of these conclusions, it is evident that species vary and develop into other forms; also that their development is progressive, as it is well known that each geological era is characterised by higher types of fauna and flora than the preceding one.

There must be a cause for this gradual development of new forms. The agent that effects these changes is, according to the Darwinian theory, natural selection.

The theory of the origin of species by gradual development, or, more properly, by descent, is often regarded as identical with, dependent on, or so closely connected with that of natural selection, as to stand or fall with it. Darwin himself, in many instances, appears to consider development by descent or evolution as almost synonymous with development by natural selection. The theory of natural selection is, however, merely a method of accounting for or explaining the gradual development of species; and it may be sound or not, without affecting the validity of the theory of descent.

It is my object in this paper to compare this theory of natural selection with that which supposes the existence of an innate force,* and to show that the latter affords the more satisfactory explanation of the principal facts of natural history, physiology, palæontology, &c.

For this purpose I will assume the truth of the theory of the origin of species by gradual development. It has been so ably defended by Darwin, in his "Origin of Species," as to leave no new argument to be urged in its favour. The greatest difficulty which it presents, and perhaps the only one of any importance, is the absence of intermediate links or gradations between existing species: this he accounts for on the theory of the survival of the fittest only in the struggle for life, and by the imperfection of the geological records.

The inadequacy of the theory of natural selection to explain all the phenomena of natural history in a thoroughly satisfactory manner has undoubtedly caused many eminent naturalists to retain their belief in the old. theory of creation.

Notwithstanding all the ingenuity of Darwin's arguments, and the vast amount of learning and scientific research he has brought to bear on the subject in support of his theory, a thoughtful reader-though much interested, and perhaps somewhat perplexed-must close his work on the Origin of Species" with a feeling of dissatisfaction; he cannot realise that such haphazard means as natural selection and the survival of the fittest can alone have wrought such marvels as are exhibited throughout creation; nor, on the primâ facie view of the case, that they alone

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*The term "Force," it must be clearly understood, is used in this paper, not in the sense of "Germ Force" or "Will Force," but to express merely a faculty of development; and that the word "growth " is used, not in the restricted sense of mere "enlargement," but in the more extended one of "progress" or "improvement."

can have built up an ever-progressing fauna and flora, infinite in variety, but always subject to definite laws, always retaining their typical characters, while evincing adaptation the most perfect to the conditions of existence.

It is not my intention to review the numberless objections to the theory of natural selection that have been urged by different naturalists, prominent among whom are Mivart, Nägeli, &c.; but to show how completely and fully the theory of an innate power of growth or expansion is borne out by the most important phenomena of morphology, and embryology taken in connection with palæontology.

In advocating the theory of expansion, I am prepared to admit that natural selection has probably been a subsidiary agent, and one of some importance; but the agency I believe to have mainly effected what Darwin ascribes to natural selection would seem to be a subtile force inherent in all organisms, too potent to be disturbed in its action to any considerable extent by extraneous influences, and destined to work certain definite results in a definite period of time; in short, a power of growth or expansion implanted in all organisms, when launched into existence, and extending through all time. This power of growth would also appear to develop in all organisms similar features, subject to the modifying effects of climate, conditions of life, &c., at the same stage of expansion.

I will consider this definition of the Innate Force in reference, first, to the phenomena of Morphology.

All plants and animals are classed according to their morphological characteristics, and not with reference to their habits, &c. The habits of different species belonging to the same class are often as different as it is possible to conceive, and they use homologous parts, that is, corresponding portions of their structures made on the same model or type for widely different purposes. That is to say, species preserve their type no matter what be their conditions of existence. Conformity to type is thus the paramount law in the formation of species; adaptation to the conditions of existence a secondary law. Darwin considers the latter law to be the higher: were this the case we should not find the former asserting itself with unvarying persistence in the most diverse adaptations of homologous parts. Type is persistent through every adaptation. Adaptation is nothing more than a modification of type to suit certain conditions. Adaptation may be the result of natural selection, or it may not; but type cannot be, as its existence is incontestibly independent of, and has no connection with,