ON THE ORIGIN OF THE HIGHER

ANIMALS.

N the study of living creatures, whether plants or animals, we begin with that which is superficial and familiar, and then gradually pass to the deeper and less known. For one who dissects out the structures, there are hundreds who observe the outward form and habits; and for one who studies the embryological development, there are numbers who dissect and study the structure of the various types in their adult condition. So that, although this biological field is as wide as the earth and as broad as the sea, yet there are very few who go to the bottom of things, working downwards, until they see the origin of a type, and then afterwards coming up to tell their less adventurous fellow-workers what facts they have found in those dark depths.

In seeking to trace the origin of organisms in the modern Darwinian manner, it is always easiest and safest to pass from the familiar to the less known, and every now and then to make a stand in the ways and to see what lies about us on this side and on that, and then to choose which way we will go, what untrodden path we will try to thread our way through. Inquirers, candid and uncandid, those who pray that they may know, and those who come fully assured beforehand that they know all about the matter already-both these sorts of inquirers ask for impossibilities; they seek to have the whole matter put into a nutshell; they cannot wait for evidence in detail. Yet the evidence of these things must come in detail or not at all.

None of those who mock shall understand; but patient, and wise, and teachable minds shall be able to learn, not adequately, indeed, but in a very useful, practical, and pleasant manner. Assuredly, the best and most laborious of the biologists of this generation, and of that which has just passed away, have not been living in the region of old3 L

VOL. XLVII.

wifedom, nor following cunningly devised fables. Men like Lyell, Darwin, and Robert Chambers, not to mention other great and cherished names, were of a sort not easily to be deceived. To say nothing of those in Europe, in America, and in the Isles of the Sea who are assured of the truth of the modern doctrine of development, we have here at home numbers of able men, each looking at the subject from a standpoint of his own, who have been convinced of the truth of this theory. There is indeed a marvellous consensus, or harmony, in the deductions of those who have been trained in these researches, and who are spending and being spent in this kind of work.

Those who know what it is to gather this excellent knowledge, who busy themselves in harvesting and garnering what Nature, in her lusty strength, has grown for them, without their sowing and without their tilling, are cheered on by the light and strength this theory gives them. These are they who, as botanists and zoologists, gather all that comes to hand, thus laying up in store all good things for the embryologist. In gathering and classifying and even dissecting the full-grown forms, they are only preparing the way, and filling the hands of the student of Development; yet there is nothing in the deductions they are able to make, that has received or that ever will receive anything but corroboration from that slower, but most important kind of work. Also those who do business in the veins of the earth, not merely near its surface, where it has been baked with frost, but deeper down; these men, who bring up the remains of old, extinct types, are ever adding to the weight of evidence in favour of this theory.

The workers of all sorts have well done what they have done, and they are a very useful and united family; but deep crieth unto deep below all that has yet been discovered, and the need for those who will go down into the very heart of things is still very great.

Now, we will suppose the candid inquirer to ask two questions; and then try to answer them according to modern lights.

1. Did the higher kinds of the vertebrata (that great sub-kingdom which is characterized by a jointed spinal column, a brain, and a spinal cord) arise suddenly, as by a creative catastrophe; or by metamorphosis of the lower kinds; or slowly, during the ages, by the accretion of gentle and easy modifications, caused by the surroundings of the creature?

2. Did the lower vertebrata arise suddenly by a creative catastrophe, or by metamorphosis of still lower, non-vertebrate types—the forms so metamorphosed subsequently undergoing slow, secular changes?

I.

The first question refers, of course, to the origin of reptiles, birds, and beasts; creatures that, from the time of their hatching or their birth, breathe air, and have no gills for aquatic respiration during any period of their life. These are the higher vertebrata. Fishes (such as the lamprey, shark, and perch) and amphibia (such as salamanders, frogs, and toads) all have aquatic respiration, either permanently or for a time. These form the lower stratum of the vertebrata.

Even in their outer clothing, the three great groups of the higher stratum-reptiles, birds, and beasts-have new and strange structures, such as are not found in the types beneath them. The exquisitely folded skin of the serpent, here wrought into parallel plaits, and there into diamond-shaped tessera; the plumage of the bird, and the hairy covering of the beast are all, in one sense, new things. They are adaptations to the new life on the dry land, in the open air. But you must have more than a hood if you wish for a monk, and the kind of clothing of these three groups is but the outside of what we have to deal with in biology.

The difficulty of supposing that the almost infinite variety of living creatures all arose from simpler, and still simpler and more generalized types, by a mere process of slow and gentle modifications, taking place during untold periods of time, is as great to the biologist as to one untrained in the science of life. To a certain extent, the old adage, nihil per saltum—nothing by leaps and startsis true in Nature; but it is not universally true. Hence no wellinformed naturalist is an absolute uniformitarian; he is also, more or less, a catastrophist. But if leaving the great difficulty of such a problem unsolved for the present--we suppose the existing groups of higher animals to have arisen from some common, low, generalized stock, then we can easily imagine the huge results that may have taken place during long, almost unlimited, secular periods. The doubter should begin by considering, first, the close relationship of the races of one type or species, and then the little, non-essential things that separate or distinguish the various species of one genus. Thus, for example, the various races of oxen (Bovidae) differ only in nonessential characters, and no one can tell where a race ends or a species begins. In this family, even the ordinary test of the fertility or non-fertility of crosses fails the naturalist altogether. Our common oxen, the bison, the aurochs, the yak, and all the different kinds of buffaloes, all go together to form one single special group, or family, in that Order of Ruminants which Moses characterizes in the following words: "Every beast that parteth the hoof and cleaveth the cleft into two claws, and cheweth the cud."

Now there are in this Order certain distinctions easily observed,

and at the same time very useful in zoology; they are derived from the most superficial modifications, from differences that are merely skin-deep. There are ruminants with hollow horns, with solid horns, and without horns. Oxen, sheep, goats, and antelopes have a hollow, bony core, covered with a horny sheath; the core is a growth from the bone of the forehead; its horny sheath is a modification of the outer skin; these horns are permanent, and are generally possessed by both sexes. In the deer family, a large branch of solid bone grows out of the forehead on each side, carrying with it the skin, which is covered with soft hair, hence called velvet. When the bone ceases to grow, the skin dies and is rubbed off against the trees. These horns, called antlers, are soon shed, and, as a rule, exist only on the male. The musk-deer, the chevrotain, the llama, and the camel have no horns of any sort. The two last kinds, the llama, and the camel, differ so much from the rest, that they form a special subdivision of the Order. They are evidently very ancient types.

Again, the larger cattle, besides being divided into ruminants and non-ruminants, are classified as even-toed and odd-toed beasts. The nobler and more modern types of even-toed beasts chew the cud; but there are some manifestly ancient forms still lingering on the planet which do not chew the cud; as, for instance, the hog, of which there are many species, and the hippopotamus. These, as is well known, like the more archaic ruminants, do not possess horns. All those beasts which have an even number of toes are destitute of the first or inner toe, corresponding to our thumb or great toe. In oxen the second and fifth toes are also suppressed, only the corresponding nails remaining as small hinder hoofs. In deer, notably in the reindeer, these hinder toes are present, but the bones are small.

As a rule, the ruminating animals have only one bone in their shank the so-called cannon-bone; but in the early embryo, this is composed of two equal parts, each of which has a convex surface for articulation with the corresponding toe-bone; this accounts for the fact that the cannon-bone carries two toes. In the nonruminating, even-toed animals-the hog and hippopotamus-these two bones never fuse to form a cannon-bone, but remain distinct; and this is seen in the fore-legs of the African water-deer (Hyomoschus -a name suggesting an intermediate position between the musk deer and the hog). This animal and its small relatives, the chevrotains of Ceylon and Java, belong to an almost extinct family of ruminants.

The hippopotamus is manifestly of an older and more general type than even the pig; he stands almost alone as the living representative of a family of gigantic even-toed beasts. In former days giants of this kind were as common as the members of the hog family are now.

None of the odd-toed cattle chew the cud; only two families. still exist-the several species of rhinoceros and the horse group, con

sisting of the horse, ass, zebra, and quagga.* The rhinoceros has three well-developed toes, each ending in a small hoof; but in the horse and his relatives only the middle toe is developed, and the bone with which this is articulated is a primarily single cannon-bone; the corresponding bone of the second and fourth digits being a mere splint, pointed below.† The rhinoceros on the one hand, and the horse on the other, are the culminating forms of the odd-toed beasts which have diverged during time into forms so remarkably unlike. It is very curious that these should be all we have left of the oddtoed herbivora.‡

And now the carnivorous tribes, the cat family, the dog family, and the kindred of the bears and seals, have all to be traced downwards to some common stock; to say nothing of aquatic whales, aërial bats, lemurs, monkeys, apes, and men. All these, in their multitudes, come flocking for the registration of their ancestry; nor do they seal up the sum of this great and varied Class, for the insectivorous kinds (moles, hedgehogs, and so forth), and the edentate tribes (the ant-eaters and pangolins with no teeth at all, and their imperfectly toothed relatives, the sloths and armadilloes), these, lowly as they are, also belong to the noble (Eutherian) types of the mammalia.

Down to this point we need ask for no catastrophe, no metamorphosis, nothing but time and surroundings, and the marvellous working of that indwelling force which moulds and fashions each type into a form in harmony with its outward life and conditions. All these types now mentioned belong to the highest of the three platforms of mammalian life; all have the common characteristic that they carry their young, and do not "cast forth their sorrows until a very considerable though varying ripeness has been attained; for a longer or shorter time they minister to the necessities of their progeny of their own substance internally, and afterwards externally, by providing them with milk.

Before I go on to speak of the creatures on the next lower platform (the Metatheria), I must remind the reader that in the groups just mentioned all our zoological distinctions fail us. As we descend

* Naturalists, as a rule, include the tapirs among the odd-toed beasts. In reality they are a much more archaic group than the rest. They possess a well-developed fifth digit on their fore-foot; only the first being suppressed.

+ Thus we see the remarkable difference in formation between the foot of a cow and that of a horse.

Amongst the herbivorous tribes just mentioned no place has been found for the huge elephant, no place for the little hyrax (dayman, or coney of the Bible); for these lie far off from the other cattle, and their kindred must be sought among the root-stocks of old and generalized types, from which sprang the forefathers of the existing rodents -the rat, squirrel, beaver, &c.

§ Eutheria (literally, "noble beasts"), Metatheria, Prototheria-the Eutheria being the placental mammals; the Metatheria the pouched animals, or marsupials; and the Prototheria those existing links which connect the Mammalian group at its lower extremity with birds and reptiles