The celebrated and various snails of the Stuben Valley, near Steinheim, in Würtemburg, whose snow-white shells constituted more than half the mass of the tertiary limestone hills, exceed twenty different species; but the extreme forms are linked by so many which are intermediate, lying regularly above and beside one another, that their pedigree is easily traced.

The historical succession is generally indicated: (1.) in the palæontological history of organisms as furnished by fossils in their adaptation to those various changes in the earth, of which increase or decrease of temperature was the master fact, affecting climate, food, land, and sea level; (2.) in the history of individual organisms; (3.) in the comparative anatomy of kindred organisms. These are the three main facts which prove that a marvellous process of adaptation has been in operation from the very beginning. For example, the Nummulites, whose shells, the size of a lentil, form whole mountains on the shores of the Mediterranean, possess a house with many little chambers artistically ordered. The Polythalamia have shell-chambers wound round one another in a spiral line. The Acyttaria possess a solid shell in great variety of exquisite forms. These little palaces of beauty, regular structure, and elegant execution, are the product of a slimy formless living mass; and various, as their products, are the builders themselves. The differences, imperceptible in their chemical composition and physical construction, are brought plainly into view by the variety of their constructed habitations.

As the chasm between creeping thing of the land and swarming thing of the sea is bridged by intermediate forms; so, between fish and animal of the land, come those amphibia of which ancient days afforded gigantic examples. Are we then to conclude that the land was colonised from the water? We may smile at those who assert that every foot comes from a fin, and that fish by gaping developed lungs; but as twice a day, in the rise and fall of the tide, some plants and animals have a twofold kind of life; those only touched by the highest tides, and those never uncovered but at lowest ebb, having intervals varying both in frequency and duration; it is easy

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to think of an advance of life from the sea to possess and replenish the land. Heredity from the water might be so acted upon through adaptation by land, that at last animals could wholly forsake the one for the other. The mud-fish is an example of transition into amphibia, and the tailed forms of amphibia are the most ancient. Tritons are amphibious animals, akin to frogs; and, like them, possess, in an early stage, gills, by means of which they live and breathe the air that is dissolved in the water. At a later stage, like frogs, they leave the water, lose their gills, and are able to breathe with their lungs; but if, by being shut up in a tank, they cannot leave the water, they retain their gills. The gilled salamander, axlotel (siredon pisciformis), generally remains all its life in the water; but at the Zoological Garden, in Paris, not long ago, a small number crept out of the water, from the many hundreds of their fellows, on the dry-land, lost their gills, and became gill-less salamanders-breathing only through their lungs.

Thus subjection to circumstances causes structural changes in the properties of already formed parts; but, within any assigned time, these changes fall within narrow limits; and so soon as the normal state is re-established the organ and organism fall back to their original state. Structure, handed down by heredity, is indeed liable to variations of considerable magnitude; in part by the individual, and in part by involved influences producing functional adaptations; but only power from without, acting within, can produce what is not inherent in the organism; and in no other way can we find a satisfactory explanation for the continual introduction throughout all geological time of new forms of life, which do not appear to have been preceded by pre-existent allied types. There exists some other deeper and far more reaching law than evolution. As to artificial acquirements, plants and animals when neglected relapse to their original wild forms; and mutilations of the bodythough continued from generation to generation, as nose and ear piercings, misshapings of the foot-as among the Chinese, and circumcision-as by the Jews, are not transmitted. However much the lower forms of life mingle; and the outward

grades of fish and amphibia, marsupial and mammal, approach one another; distinct provinces of marine and terrestrial life are always maintained.

The Marsupial form, so akin to the reptile, preceded the other Mammalia in time. Of the Mesozoic species all are marsupial, small, and of low grade; but not lower than some now existing. Their low position may be associated with the habit of limiting the exercise of active-life faculties to the period of night's obscurity. The mother nurses her young in a tegumentary pouch, where they remain suspended to the teats, and are safely carried for a period answering to that of uterine life in the higher mammals. Where great want of water exists; and, in dry seasons, rivers are converted into pools few and far between; in such a climate and at such a time, an ordinary non-marsupial animal, like the wild cat or fox, having deposited her young, would travel perhaps one or two hundred miles to quench her thirst. "Before she could return her blind and helpless litter would have perished. By the marsupial modification the mother is enabled to carry her offspring with her in the long migrations necessitated by the scarcity of water."" Mr. Owen adds, "These correlated modifications of maternal and foetal structures, designed with special reference to the peculiar conditions of both mother and offspring, afford, as it seems to me, irrefragable evidence of creative foresight." A difficulty attends this theory: the male has a pouch, and those which live in trees and near water.

The great class of Mammals had a small beginning, and made little or no advance during the vast Mesozoic time. In the Tertiary or Mesozoic time existed that higher group which now has pre-eminence: animals related to the tapirs, bears and racoons are among the oldest. The Miocene was the culminating age of Mammalia; they were then largest and most numerous. The Deinothere was as much larger than our elephant as the elephant exceeds an ox; the skull, including snout, five or six feet long; two large tusks grew out of the end of the lower jaw and pointed downwards. The

1“Classification and Geographical Distribution of the Mammalia:" Richard Owen, F.R.S.

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most ancient beasts of prey are the feline, then the canidæ, and latest the ursidæ. Relics of their predecessors we do not possess. The beast of the earth and cattle are the freely roving vegetable and flesh-eating wild animals of the land, creatures of the marsh, the field, the forest, and the plain. The Eocene and Miocene strata of North America are crowded with carnivora, ruminantia, pachydermata, rodentia, and non- ruminating creatures of the horse, rhinoceros and pig tribes.

Heredity, or inheritance of the parents' nature, seems to be the natural cause of stability; and Adaptation the cause of modification or change in organisms. Structure and function being exposed to countless actions and reactions, from generation to generation, of ever varying circumstances, the wonder is that we have not greater varieties, and that they are not mingled in utter confusion. There must be some deep and far-reaching law marvellously adjusting stability and instability, multiplication and extinction, that due equilibrium may be preserved. In every species, animal or vegetable, the individuals are never quite alike; and in every species, even in every individual, there is a tendency to differences great enough to produce varieties. Some unknown energy sets bounds to these changes; and we are amazed that the simple egg-cell of the maternal organism, and a single paternal sperm-thread, transfer to the young the minutest bodily and mental peculiarities of both parents. The germ from which most mammals are produced is the 120th part of an inch in diameter, the same size as in man.

Doubtless, we may say of life-vires acquirit eundo. The original forms of it, whether few or many, were capable of development, and received it. Every new natural principle was to the preceding as a miracle, the animal a miracle to the vegetable, and man a miracle to the beast of the field. Life did not flow in an organic circle; some forms, for reasons unknown, being retarded; and other forms, ascending in many lines of development, brought in new things. Every species seems to come into being at a certain definite time, and to disappear at another definite time, though there are few, if indeed any instances, in which we can safely fix the time for entrance or exit. This bringing in or creating of new things,

is our general notion of a miracle; and foretelling new things is our general idea of prophecy. All past variety of growth and development of power were figures of future advance, or allegory of forms to come; every low grade reappearing in the higher, as initial, subservient and supporting substance. On natural life is grafted intellectual life; on intellectual life, the spiritual and moral life; on spiritual and moral life, future life. Life, ascending in many various paths, is everywhere subjected to spirit; and life subjects and connects matter, as the crystallographer connects imponderable forces and polarity; the coarse or outside substance becoming, so to speak, the precipitate of inner and finer formations. There are worlds within worlds; infinity contains space, space comprehends matter, matter embraces life, life enfolds intelligence, intelligence is the breath of spirit.

Life-forms are classed according to the differences in structure. Heredity tends to conservation; and power of Adaptation, by circumstances to circumstances, tends to variety. Hence, offspring resemble their parents; but are never wholly alike either in form or in structure. The likeness preserves the identity of species, unlikeness tends to variety. Sometimes the variety arises in full force, per saltum; but in every case there are determining causes, external or internal, or both. Even a variety which approaches the nature of a monstrosity strives to reproduce itself; much more those which are better fitted to maintain the struggle for existence. Natural causes, agents of Divine or Unknown Energy, acting through long ages of time, brought into existence from pre-existing life-forms all the varieties of life now on our globe; there is the constant introduction, throughout geological time, of new forms of life which do not appear to have been preceded by pre-existent allied types;1 the weaker, perishing; those excelling in strength, skill, agility, and best fitted to surrounding conditions, surviving. As a rule, the animals of lowest and simplest organisation have the longest range of time; the additional possession of minute dimensions is also in favour of their continuance.

1 For examples, see H. Alleyne Nicholson's "Life History of the Earth,” P. 373.