6 MAY 1871, Page 14

BOOKS.

THE GENESIS OF SPECIES.*

Tars Origin of Life, the Genesis of Species, the Descent of Man— such are the grand problems which now-a-days engage the atten- tion of philosophic biologists. No longer is the mere description. and collection of plants and animals, the ransacking of sea and land, and ancient rocks for the countless forms of past and present lifer the chief object of our students of botany, zoology, and pa!monto- logy. Nor are they satisfied with the study of the geographicab distribution, the succession in time, or the individual developmeut. of vegetable and animal organisms. The newer modes of iuvesti- gation do indeed demand the most profound knowledge on all these points and the most extensive collections of materials, but. they ask much more. They ask, especially, for a wider view or the relation of living things both to the organic and inorganic worlds ; yet they employ, as one of the most efficient instruments- of research, a minute scrutiny into the causes of the most trifling peculiarities of form and colour. Whatever be the ultimate out- come of this novel questioning of nature, and in whatever way we- may have to interpret its revelations, one thing is certain, that we- owe to it many discoveries of surpassing interest. Work of this- kind has been greatly stimulated by the publication of the views- of Mr. Darwin, and Mr. Wallace, on the theory known as that of "Natural Selection." This really ancient theory, revived by Dr.. Erasmus Darwin in the last century, and marvellously developed. by his grandson within the last ten years or so, is discussed by Mr.. Mivart in the volume before us. This book was, however, pub- lished before the appearance of Mr. Charles Darwin's last work, in which he gives his present views on natural selection as related, * On the Genesis qf Specie.% By St. George idivart, F.E.S. London : Macmillan- 1871.

to the descent (or, as it has been more appropriately named, the ascent) of man.

To refute or to urge the adoption of the theory of natural selection is not Mr. Mivart's object. He desires to discover its Teal value, and to help in assigning to it a proper place in the -scheme of nature. Farther than this, he desires to conciliate the advocates of opposing theories, and to "dig down to the common root which supports and unites diverging stems of truth." More- -over, he-endeavours to show that the theory of the evolution of a series of organic forms is not antagonistic to the teachings of even "the most orthodox theology. Before proceeding to give some notion, however imperfect, of the plan which Mr. Mivart has pursued in dealing with these subjects, it will be necessary to state, though very briefly, the main positions assumed by those biologists who advocate the Darwinian doctrine of natural selection.

Starting from the fact that every kind of animal and plant tends to increase in numbers in a geometrical progression, and transmits a general likeness with minute individual differences to its offspring, natural selection assumes that such of these minute 'differences as are in any way beneficial to the individual possessing them become in the long lapse of ages more and more pronounced. This accumulation of peculiarities or divergencies from the parental type is caused by the "survival of the fittest" in the struggle for existence which all individuals have to en- -counter, and results in the production of new kinds or species. So far natural selection takes into account utility, only supposing "the preservation and development of such differences and variations -as are simply beneficial to the individual. But selection may be -exercised, we are told, in another direction, that of beauty ; and so, for example, a bird having exceptionally brilliant plumage may more readily find a handsome mate, and so perpetuate a race -of increasing beauty. A further supplement to the theory of natural selection is afforded by Mr. Darwin's doctrine of Pan- genesis, by which he endeavours to account for the likeness of progeny to parent through generation after generation. Of this theory we shall speak further on.

Mivart allows the existence of such an agent as natural selection, and even points out how it may serve to explain some of the most difficult phenomena of organic forms, but he draws a boundary to its efficient action. He regards it as wholly inapplica- ble or quite insufficient to explain the production of a very large slumber of existing animal and vegetable structures. His argu- xnents and illustrations are drawn from all three kingdoms of nature, but he is obviously most at home when dealing with .zoological matters. In two chapters the main difficulties of Darwinism are arranged and discussed, the remainder of the book being occupied by a summary of the preceding arguments, by a brief statement of the author's own views, and by an inquiry into the relation between certain theories of organic evolution and the 'Christian theology. We will first cite a few of the difficulties -which prevent Mr. Mivart's complete acceptance of the theory of natural selection.

The first difficulty consists in accounting for the preservation of

.what may be called the incipient stages of useful peculiarities. The neck of the giraffe, as it now exists, is doubtless useful to these animals in times of drought, enabling them to crop the foliage of -trees when the ground vegetation is wholly burnt up. But of what use would have been an excessively minute elongation of the neck in the nascent giraffe species ? What advantage would such a minute peculiarity, such an infinitesimal lengthening of the neck, as Darwinism supposes, bring to the animal possessing it? Could it have tended to the preservation and predominance of this slight 'deviation from the original type, while all the individuals of the ancient and short-necked form were destroyed during each suc- cessive time of drought ? So, too, with respect to the eyes of soles and other flat fishes. It is well known that in these animals both -eyes are situated, in the adult, on the same aide of the head. If 'this transference of an eye was accomplished by imperceptible gradations, through long ages, what benefit could the individuals in which it first occurred have derived from this divergence from a symmetrical arrangement? Convenient when perfect, the imperfect transference of the eye could hardly have been an improvement. Amongst insects numerous cues have been observed of striking resemblances to other objects. Some butterflies resemble dead leaves, and undoubtedly owe their preservation from insectivorous birds partly to this cause. So also there are walking-leaf and stick insects, and others simulating all sorts of out-of-the-way and bizarre objects. Mr. Mivart asks how is it that a number of minute incipient variations in many direc- tions can ever build up such a resemblance. to a leaf or a bamboo as to deceive not only an insectivorous bird, but a shrewd naturalist ? How could natural selection find the materials to work upon, to perpetuate, and to perfect, out of the very small and oscillating beginnings of such a resemblance? For the resem- blance ought to be in one direction only, and for its perpetuation many individuals must be simultaneously modified in that par- ticular direction. But perhaps a still greater difficulty is experienced when, leaving the incipient stages, we endeavour to account, by natural selection, for the last finishing touches of these extraordinary mimicries. It seems impossible to see how the "survival of the fittest" can explain the imitation, by insects, not only of leaves, but even of the powdery holes and mildews that attack these leaves, and of their other injuries. Many other illustrations of the difficulty which exists in accounting for the preservation and development of useful structures in their incipient stages are fully discussed by the author, but we cannot reproduce here the elaborate arguments necessary to convey his meaning. We may, however, refer to the cases of the expanding neck of the cobra and the rattle of the rattlesnake, both of which, by warning through eye or ear the intended victims, seem posi- tively injurious to the animals possessing them. For the timid and cautious only of the animals on which these snakes prey would ultimately survive, and would inherit a tendency to distrust "ex- panding" and "rattling" snakes, so that the latter would experience increased difficulty in obtaining their supplies of food. A very important argument, drawn from the minute anatomy of the eye and ear, is included in chapter ii., the independent but simul- taneous processes by which these organs are developed being proved inexplicable on the natural-selection theory, with its minute and gradual modifications, since the incipient stages of these processes are in themselves absolutely useless for sight or hearing.

The co-existence of very similar structures of diverse origin tells against the sole or chief influence of natural selection in the pro- duction of such structures. Many causes beside natural selection do indeed produce these concordant variations. Among such causes may be named climate, soil, and food, which often tend to impress a peculiar character upon the forms, structure, and colouring of the plants and animals of particular regions. The faunas and floras of many islands may be adduced to show how important physical causes are in favouring particular modifications. The generally succulent and fleshy leaves of glaucous hues, which many inland plants acquire when grown near the sea, and which sea-shore plants so commonly possess, may be cited as a case in point.

There is an argument against the gradual origination of new species by the long accumulation of minute changes furnished by the sadden modification which animals have been known to exhibit, especially under altered outward conditions. A cat from Algoa Bay underwent a complete metamorphosis after having been left only eight weeks at Mombas (on the African coast), where the cats, instead of fur, have short stiff hairs. So, too, some greyhounds of the best breed, imported from England for hare- hunting in a mountainous district of Mexico, were found quite useless, not being able to support the fatigue of the chase in the attenuated atmosphere. Yet the direct progeny of these animals was so changed as not to be incommoded by the want of density in the air. But the most cogent argument urged by Mr. Mivart against gradual modifications is supplied by the sudden appearance and steady persistence of a new and well-marked variety or form of . some animal or plant. Per- haps the five distinct developments of the " black-shouldered " or "japanned" peacock in flocks of ordinary peacocks is the most extraordinary instance of this kind. Not only did this form spon- taneously appear in Sir J. Trevelyau's flock, previously entirely of the common kind, but it so increased as to exterminate the pre-

viously existing breed. What renders this case the more remark- able is that the japanned peacock is regarded by one of the best ornithologists of the day as a distinct species, not a mere variety.

The relation of the fixity of species to natural selection is the subject of chapter v. Had such a law of selection been in con-

stant and general operation, the present distinctness of species, and marked difference between species and races, could not have been preserved. Nor could the limits of variation in races or varieties have been so soon reached as they are found to be practically. Then, too, the general sterility of hybrids militates against the assumed unreality of species.

There are two modes in which species may be regarded in their relations to time. If species have developed through successive increments of minute variations, we ought to discover in the stone records of the earth's history very distinct evidence of transitional and intermediate forms. Not only so, but the number of such.

progressive steps having been innumerable, the intermediate forms should be discovered in preponderating numbers. Why do not

such forms exist? Why do they not exist in countless specimens?

But, in point of fact, many fossils known to be continued during or separated by long periods of geological time are found to be absolutely identical in every respect ; while many of the varieties which do occur, occur in the same stratum, perhaps in contact, and present just such " reverting " variations as are found in existing individuals of the present day. Mr. Mivart says, "It is incredible that birds, bats, and pterodactyls should have left the remains they have, and yet not a single relic be preserved, in any one instance, of any of these different forms of wing in their incipient and relatively imperfect functional condition !" The other point in which the element of time has to be taken into account in speaking of the slow de-

velopment of specific forms, involves the question how long a period is demanded for this purpose by the tedious processes

of natural selection ? Now, Sir W. Thompson has endeav- oured to ascertain, by three chains of reasoning, astronomical and physical, how long life can have existed upon the earth.

And he concludes that all geological history showing continuity of life must be limited within a period of one hundred million years. Mr. Mivart, with many other biologists, considers this period far too short for the multifarious developments, by exceedingly minute fortuitous variations, of the species of past and present time. He points out the present stability of animal forms, as represented on ancient monuments, and as occurring in what are known as recent deposits. He then proceeds to show how vast a period, some two thousand million years, must in reason and by analogy be assigned to the differentiation of a certain species of animal of the genus to which it belongs, of the family which includes its genus, and to the scores of earlier differentiations which must have preceded those just mentioned, and which lead us back at last to a worm or a jelly-fish !

The questions of the geographical distribution of plants and animals are discussed in chapter vii., while chapter viii. endeavours to show that natural selection is incapable of accounting for the phenomena of homology, that is, the common resemblance of parts in their relations to surrounding parts, or as to their mode of origin. Why, for example, should most wonderfully complicated but quite useless structures be produced, and perpetuated in all their perfection, merely in order to preserve the symmetry of the form and its typical relations ? Indefinite variation with the sur- vival of the fittest does not seem to be in any way concerned with the elaboration and preservation of special organizations of this kind, which should rather be traced to the conjoint effects of seve- ral causes. Amongst these, an innath tendency, however inexplic- able, holds the first place, just as a crystal is built up into a definite and constant form owing to the innate tendency of its constituents, when suitable outward conditions are fulfilled.

Pangenesis is a supplementary theory to that of natural selec- tion. It has been devised to account for heredity, of the like- ness of progeny to parent. It is, in fact, a means of localizing the innate tendencies of living forms. In reality it involves the assumption it was intended to supplant. For it supposes the existence in every living organism of a vast number of circulating particles, termed gemmules, each capable of reproducing its kind, and derived from all parts of the less remote ancestors of each such organism during all the states of such ancestors' existence. Here, then, we have the hereditary characters of the organism transmis- sible bit by bit through the power or tendency of gemmules instead of through the power or tendency of those parts of the organism capable of reproducing it. Endeavouring to dispose of this existent innate and directive tendency inherent in organisms, whereby they produce their like, pangenesis imagines countless generations of gemmules, it endows them with an imaginary pro- pagative power, and it suggests that they, though atomic, are susceptible of multiplication by fission.

We have not space to offer a summary of Mr. Mivart's positive teaching on the subject of Specific Genesis. So far as his views have taken a definite shape, he is inclined to attribute great power in the origination of new forms to an internal tendency, real, but as yet inexplicable. Still he allows external conditions, with inheritance, reversion, natural selection, &c., to play an important part in the work. Natural selection more particularly is concerned in the suppression of monstrosities, as Aristotle taught,

and feeble and abortive forms, removes antecedent species when the new one which is evolved is a start, as it were,—is more in harmony with existing conditions, and finally, favours useful -variations.

We wish we could present our readers with the arguments of

Mr. Mivart's twelfth and final chapter, in which he attempts the reconciliation of science and religion as regards the doctrine of evolution of species. But it would be unfair both to the author and the subject to attempt to contract his chain of reasoning into a few short sentences at the end of a notice like the present. We. can only say that his great knowledge, and temperate, though_ forcible language, combine to render his views as to the bearings of science on theology and morality (pp. 188 to 207 and 243 to 288). deserving of the most attentive perusal.