appeared, and the lower orders of being, which formerly occupied the scene, have entirely died out, so that their remains, entombed in the solid rock, are now the only indications of their past existence. In the era of the primary rocks, as we have seen, there was no organization or life, as there was nothing to support it. In the succeeding period, zoöphytes and mollusca appeared; these were followed by fishes, and then land rose above the surface of the waters. Land-plants and animals came next, though of a low type; continually advancing orders of beings, reptiles, birds, and mammifers, suited to the improved condition of things, successively appeared, until, at the latest epoch, man entered upon the scene, the head of animated nature as at present constituted, with powers and capacities well adapted for the full enjoyment of the augmented riches of the earth. And the end is not yet. "The present race, rude and impulsive as it is, is perhaps the best adapted to the present state of things in the world; but the external world goes through slow and gradual changes, which may leave it in time a much serener field of existence. There may then be occasion for a nobler type of humanity, which shall complete the zoölogical circle on this planet, and realize some of the dreams of the purest spirits of the present race."

The question now occurs, How are we to account for the origin of life, both in the vegetable and animal kingdoms? The answer can readily be given, if we follow out resolutely to their remotest consequences the principles that have already been established. The evolution of natural laws, the necessary action of the qualities with which atoms were at first endowed, has sufficed to produce this complex system of mutually dependent worlds, and all the successive transformations of the earth's rind, which have fitted it for the support of successive races of organic beings. May not the same causes have produced the beings themselves? The one process would seem to be not much more elaborate and intricate than the other. If the inherent qualities of matter have built up a solar system, they may have created, also, the first animalcule, the first fish, the first quadruped, and the first man. There has been a marked progress, in either case, from the chaotic, the rude, the imperfectly developed, up to the orderly, the complex, the matured forms. The first essays, the rude efforts, of nature have gradually been perfected. The chaotic world that was first shelled off from the sun differed not less widely from the admirably furnished planet we now inhabit, than does the zoöphyte, whose remains are not split out of the rock, from man, the present head of the animal tribe. At any rate, geology informs us, that the causes, whatever they may be, which produce life, have been long and frequently in operation. They were not exhausted in the first effort; they are probably still at work throughout the universe. Not merely successive generations, but successive races, both of plants and animals, widely distinguished from each other, have, at different periods, tenanted the earth's surface. Those of which we possess the fossil remains belong, almost without exception, to extinct species. They were crowded out of existence, as it were, by the new forms, more perfectly organized, which came to take their places in the improving condition of things. This continuous agency of the life-producing causes, effecting still higher results by each successive effort, seems to point directly to the gradual expansion and development of the qualities with which matter was first endowed.

We actually see natural agents now at work around us, producing results which counterfeit life, if they do not constitute it. Many substances crystallize into shapes bearing a strong resemblance to vegetable forms, as in the well known chemical experiment producing the arbor Dianæ. The passage of the electric fluid leaves marks that are like the branches and foliage of a tree, and the same fluid exerts a direct influence on the germination of plants. Some of the proximate principles of vegetable and animal bodies, such as urea and alantoin, are said to have been produced artificially by the chemist; and in the combination of the simple elements, such as carbon and oxygen, into these proximate principles, it is now acknowledged that there is no violation of the ordinary laws of chemical affinity. The origin of all vegetable and animal life, so far as it can be traced, is in germinal vesicles, or little cells containing granules. Such are the ova of all animals; and both vegetable and animal tissues are entirely formed from them. When the parent cells come to maturity, they burst and liberate the granules, which immediately develope themselves into new cells, thus repeating the life of their original. Now, it has been asserted, that globules can be produced in albumen by electricity; and if these globules are true germinal vesicles, the difficult problem of producing life by artificial means is entirely solved.

But the burden of this part of the theory rests on the evidence that has been produced of late years to favor the doctrine of equivocal generation, or the production of living beings without the agency, either direct or indirect, of parents of the same species. Can such beings, orphans in the strictest sense, now be produced or discovered? We have not space to repeat our author's argument on this difficult mooted question in science, nor is it necessary; he sums up the evidence on his own side, and of course finds it satisfactory, though the weight of authority is against him. He adduces the experiments of Mr. Crosse, repeated by Mr. Weekes, who claim to have produced animalcules in considerable numbers, of a species before unknown, by passing a voltaic current through silicate of potash, and through nitrate of copper. The existence of entozoa, or parasitic animals, found in the interior of the bodies of other animals, and found nowhere else, is thought to support the same doctrine. The question is, How came they there? Being too large, either in their perfect form, or in the egg, to have passed through the capillary blood-vessels, how came they within the body of another animal,—itself but a few weeks or a few days old, or even in the embryo stage,—unless they were created there without parentage of their own species?

These facts and reasonings, it is true, only go to prove, that animalcules, or beings of very small size, and low in the scale of animated existence, can be produced in this way by the inherent qualities of matter. No one will pretend, that a dog, a horse, or a man can thus be created. How can we account for the existence of these larger animals of a higher type, admitted to have been denizens of the earth only since the latest geological epochs, and therefore of comparatively recent origin? Here we come to another point in our author's theory,—the transmutation of species, or the successive development of higher and higher orders of being out of the species immediately below them, through the accidental or natural fulfilment of certain conditions, in the course of a long period of years.

Natural history teaches us, that there is quite a regular gradation among the several tribes of vegetables and animals; though we may not be able to range all the species, as constantly advancing in a single line, there is certainly the general appearance of a scale, beginning with the most simple, and going on to the most complex forms. While the external characteristics are very different, all are but variations of a single plan, which exists as the basis of all, and is varied in each individual only so as to accommodate it to the conditions under which the individual is to live. The germ of a higher animal—a mammifer, for instance—is the representative of a lower animal full-grown, like the volvox