been that which led to the establishment of the greater part of our freshwater fauna. Professor Sollas has shown with much conclusiveness that the conversion of comparatively shallow continental seas into freshwater lakes has taken place on a large scale several times in the history of the earth. This has been in all likelihood accompanied by the transformation of marine into freshwater species. It is thus, we believe, that our lakes and rivers were first peopled. Many freshwater forms differ from their marine relatives in having suppressed the obviously hazardous free-swimming juvenile stages, in bearing young which are sedentary or in some way saved from being washed away by river currents. Minute and lowly, but marvellously entrancing, are numerous Rotifers, of which we know much through the labours of Hudson and Gosse. These minute forms are among the most abundant tenants of fresh water, and their eggs are carried from one watershed to another on the wings of the wind and on the feet of birds, so that the same kinds may be found in widely separate waters. Let us see them in the halo of Hudson's eulogy: "To gaze into that wonderful world which lies in a drop of water, crossed by some atoms of green weed; to see transparent living mechanism at work, and to gain some idea of its modes of action; to watch a tiny speck that can sail through the prick of a needle's point; to see its crystal armour flashing with ever-varying tints, its head glorious with the halo of its quivering cilia; to see it gliding through the emerald stems, hunting for its food, snatching at its prey, fleeing from its enemy, chasing its mate (the fiercest of our passions blazing in an invisible speck); to see it whirling in a mad dance to the sound of its own music-the music of its happiness, the exquisite happiness of living,-can any one who has once enjoyed this sight, ever turn from it to mere books and drawings, without the sense that he has left all fairyland behind him?" Not less lively than the Rotifers are crowds of minute crustaceans or water-fleas which row swiftly through the clear water, and are eaten in hundreds by the fishes. But there are higher forms still : crayfish, and the larvæ of mayflies and dragonflies, mussels

and water-snails, fishes and newts, the dipper and the kingfisher, the otter and the vole.

As we review the series of animals from the simplest upwards, we find a gradual increase in the number of those which live on land. The lowest animals are mostly aquatic—the sponges and stinging-animals wholly so; worm-like forms which are truly terrestrial are few compared with those in water; the members of the starfish group are wholly marine; among crustaceans, the woodlice, the land-crabs, and a few dwellers on the land, are in a small minority; among centipedes, insects, and spiders the aquatic forms are quite exceptional; and while the great majority of molluscs live in water, the terrestrial snails and slugs are legion. In the series of backboned animals, again, the lowest forms are wholly aquatic; an occasional fish like the climbing-perch is able to live for a time ashore; the mud-fish, which can survive being brought from Africa to Europe within its dry "nest" of mud, has learned to breathe in air as well as in water; the amphibians really mark the transition from water to dry land, and usually rehearse the story in each individual life as they grow from fish-like tadpoles into frog- or newt-like adults. Among reptiles, however, begins that possession of the earth, which in mammals is established and secure. As insects among

the backboneless, so birds among the backboned, possess the air, achieving in perfection what flying fish, swooping treefrogs and lizards, and above all the ancient and extinct flying reptiles, have reached towards. Interesting, too, are the exceptions-ostriches and penguins, whales and bats, the various animals which have become burrowers, the dwellers in caves, and the thievish parasites.

But it is enough to emphasise the fact of a general ascent from sea to shore, from shore to dry land, and eventually into the air, and the fact that the haunts and homes of animals are not less varied than the pitch of their life.

3. Wealth of Form.--As our observations accumulate, the desire for order asserts itself, and we should at first classify for ourselves, like the savage before us, allowing similar impressions to draw together into groups, such as

birds and beasts, fishes and worms. At first sight the types of architecture seem confusingly numerous, but gradually certain great samenesses are discerned. Thus we distinguish as higher animals those which have a supporting rod along the back, and a nerve cord lying above this; while the lower animals have no such supporting rod, and have their nerve-cord (when present) on the under, not on the upper side of the body. The higher or backboned series has its double climax in the Birds and the furred Mammals. Indissolubly linked to the Birds are the Reptiles,—lizards and snakes, tortoises and crocodiles--the survivors of a great series of ancient forms, from among which Birds, and perhaps Mammals also, long ago arose. Simpler in many ways, as in bones and brains, are Amphibians and Fishes in close structural alliance, with the strange double-breathing, gill- and lung-possessing mud-fishes as links between them. Far more old-fashioned than Fishes, though popularly included along with them, are the Round-mouths-the halfparasitic hag-fish, and the palatable lampreys, with quaint young sometimes called "nine-eyes." Near the base of this series is the lancelet, a small, almost translucent animal living in the sea-sand at considerable depths. It may be regarded as a far-off prophecy of a fish. Just at the threshold of the higher school of life, the sea-squirts or Tunicates have for the most part stumbled; for though the active young forms have been acknowledged for many years as reputable Vertebrates, almost all the adults fall from this estate, and become so degenerate that no zoologist ignorant of their life-history would recognise their true position. Below this come certain claimants for Vertebrate distinction, notably one Balanoglossus, a wormlike animal, idolised by modern zoology as a connecting link between the backboned and backboneless series, and reminding us that exact boundary-lines are very rare in nature. For our present purpose it is immaterial whether this strange animal be a worm-like vertebrate or a vertebrate-like worm.

Across the line, among the backboneless animals, it is more difficult to distinguish successive grades of higher

and lower, for the various classes have progressed in very different directions. We may liken the series to a school in which graded standards have given place to classes which have "specialised" in diverse studies; or to a tree whose branches, though originating at different levels, are all strong and perfect. Of the shelled animals or Molluscs there are three great sub-classes, (a) the cuttlefishes and the pearly nautilus, (b) the snails and slugs, both terrestrial and aquatic, and (c) the bivalves, such as cockle and mussel, oyster and clam. Simpler than all these are a few forms which link molluscs to worms.

Clad in armour of a very different type from the shells of most Molluscs are the jointed-footed animals or Arthropods, including on the one hand the almost exclusively aquatic crustaceans, crabs and lobsters, barnacles and "water-fleas," and on the other hand the almost exclusively aerial or terrestrial spiders and scorpions, insects and centipedes, besides quaint allies like the "king-crab," the last of a strong race. Again a connecting link demands special notice, Peripatus by name, a caterpillar- or worm-like Arthropod, breathing with the air-tubes of an insect or centipede, getting rid of its waste-products with the kidneys of a worm. It seems indeed like "a surviving descendant of the literal father of flies," and suggests forcibly that insects rose on wings from an ancestry of worms much as birds did from the reptile stock.

Very different from all these are the starfishes, brittlestars, feather-stars, sea-urchins, and sea-cucumbers, animals mostly sluggish and calcareous, deserving their title of thorny-skinned or Echinodermata. Here again, moreover,

the sea-cucumbers or Holothurians exhibit features which suggest that this class also originated from among "worms."

But "Worms" form a vast heterogeneous "mob," heartbreaking to those who love order. No zoologist ever speaks of them now as a "class"; the title includes many classes, bristly sea-worms and the familiar earthworms, smooth suctorial leeches, ribbon-worms or Nemerteans, round hairworms or Nematodes, flat tapeworms and flukes, and many

others with hardly any characters in common. To us these many kinds of " worms are full of interest, because in the

past they must have been rich in progress, and zoologists find among them the bases of the other great branchesVertebrates, Molluscs, Arthropods, and Echinoderms. "Worms" lie in a central (and still muddy) pool, from which flow many streams.

Lower still, and in marked contrast to the rest, are the Stinging-animals, such as jellyfish throbbing in the tide, zoophytes clustering like plants on the rocks, sea-anemones like bright flowers, corals half-smothered with lime. In the Sponges the type of architecture is often very hard to find. They form a branch of the tree of life which has many beautiful leaves, but has never risen far.

Beyond this our unaided eyes will hardly lead us, yet the pond-water held between us and the light shows vague specks like living motes, the firstlings of life, the simplest animals or Protozoa, almost all of which have remained mere unit specks of living matter.

It is easy to write this catalogue of the chief forms of life, and yet easier to read it to have the tree of life as a living picture is an achievement. It is worth while to think and dream over a bird's-eye view of the animal kingdom to secure representative specimens, to arrange them in a suitably shelved cupboard, so that the outlines of the picture may become clear in the mind. The arrangement of animals on a genealogical or pedigree tree, which Haeckel first suggested, may be readily abused, but it has its value in presenting a vivid image of the organic unity of the animal kingdom.

If the catalogue be thus realised, if the foliage come to represent animals actually known, and if an attempt be made to learn the exact nature, limits, and meaning of the several branches, the student has made one of the most important steps in the study of animal life. Much will remain indeed-to connect the living twigs with those whose leaves fell off ages ago, to understand the continual renewal of the foliage by the birth of new leaves, and finally to understand how the entire tree of life grew to be what it is.