marked way from the fauna of the other half of the globe. For instance, the rhinoceros, the hippopotamus, the giraffe, the antelope with undivided horns, the hedgehog, the mole proper, are only inhabitants of the Old World, whence also the horse originally came, the striped ones in Africa and the non-striped in Asia; on the other hand, the lemur, the ant-eater, the armadillo, and others, are limited to South America. The apes of the Old World have five molar teeth on each side of the jaw, narrow noses, tails usually short and never prehensile, and fleshy protuberances for sitting; the apes of the New World have six molar teeth, flat noses, and long prehensile tails. And on the contrary, where closely related species are found on both parts of the globe, they belong only to genera of which single species live or have lived in the far North; as, for instance, the rein-deer, still common to the Old and the New World in this very North which once formed a bridge between the two halves of the earth. The same is true in regard to cattle, the deer, the cat, the dog, the hare. Similar facts can also be shown of other animal classes. The farther the different species of these genera withdraw from the North Pole, the greater become the differences between the species on the one half of the globe and the analogous species of the other. Compare on this point K. E. von Baer's "Studien aus dem Gebiete der Naturwissenchaften, über Darwin's Lehre," ("Studies from the Realm of Natural Science upon Darwin's Teachings"), p. 356 f. If we add, further, the before mentioned fact, that those genera which are exclusively peculiar to one or the other continent, have their related predecessors in the tertiary strata of these continents,
the hypothesis of a separate origin for each single species, without genealogical connection with the anatomically and physiologically related species, becomes neither more nor less than a scientific impossibility.
Moreover, there are several facts of comparative anatomy which have long been the joy of all zoölogists and have rewarded the toilsome labors of detailed investigations by a delightful view over the whole realm of the organic world, but which find a scientific explanation only in the descent theory. They are the homology of the organs, and to a certain degree also the so-called rudimentary organs. By homology of organs we mean the fact that within one and the same class-group of organisms all the organs, and especially the organs in their most solid constituents, in the skeleton, are built after one and the same fundamental plan, and therefore are even in their most widely separated modifications varied after this one and the same plan. This is especially true of the vertebræ and the limbs. This homology goes so far within one class, particularly within the class of mammalia, that, for instance, the hands and feet of man, the hands of the ape, the paws of the beast of prey, the hoof of the horse and of the ox, the paws of the mole, the fins of the seal and of the whale, the wing-membranes of the flying-squirrel, correspond to one another in their smallest parts and ossicles, and can all be registered with the same numbers and letters; i.e., they are homologous to one another even to the minutest detail. The ideal plan and connection in the organisms, disclosed by these facts, and long ago acknowledged and admired, receives at the same time its simple material basis through the acceptance of a common descent.
A similar relation is observed in rudimentary organs.
Many of them, as the nipples of males, point, if not to a common descent from a lower form, at least to a common plan of the sexes. But when the embryo of the whale still has its teeth in the jaw, the grown up whale its hip-bones, when the eye of man still has its winking membrane, the ear and many portions of the skin their rudimentary muscles of motion, the end of the vertebral column its rudimentary tail, the intestinal canal its blind intestine; when sightless animals, living in the dark, still have their rudimentary eyes, blind worms their shoulder-blades; when in like manner the plants, especially in their parts of fecundation, show in great number such rudimentary organs as are entirely useless for the functions of life, but which are never misleading in determining their relationship with other plants:—how simply are all these facts explained by the descent theory, how not at all without it!
Finally, if we now mention the history of the development of animals, we shall have to postpone to the next section the consideration of the most essential facts furnished by this science; for the individual development of animals is a process which could speak not only for a descent of the species, but also for a descent of them through gradual development. But where, as in the present section, we treat the descent theory apart from the evolution theory, we have also to think of the possibility that the species or groups of species are not originated through gradual development, but nevertheless do originate through descent—namely, in leaps through metamorphosis of germs or a heterogenetic generation; and for such an idea we find confirmation in the
observation of the history of development of animals, which we call change of generation or metagenesis.[[4]] By this is meant the following phenomenon: Certain animals, as the salpa and doliolum of the order of the tunicata, as well as certain mites and many tape-worms, produce offspring which are wholly dissimilar to the mother stock. These offspring have the capacity of reproducing themselves—if not by sexual means, as at the first generation, still by the formation of sprouts; and it is only the animals originated by the second generation (with many species, even those by the third) which return again to the form of the first generation. The plant-lice transmit themselves through six, seven, even ten generations by means of sprouts, until a generation appears which lays eggs. Now it is indeed true that the change of generation forms a circle in which the form of the last generation always returns to that of the first, and therefore leaves the species, as species, wholly unchanged. But it is nevertheless a process which shows that the natural law of an identity between generator and product, observed in other relations, is not without exception; and if we once have reason to suppose that the generation of new species took place in past periods of the globe, but has ceased in the present, such processes in the single period open to our direct observation—namely, the present (in which, however, according to our knowledge, the species remain constant)—are
nevertheless hints worthy of notice. For they refer us to ways in which in those former times, when certainly new species did originate, this formation of species might possibly have taken place.
This consideration leads us to treat of the main objection raised to every descent theory: namely, that never yet has the origin of one species from another been observed, but that, on the contrary, all species—so far as our experience goes, stretching over thousands of years—remain constant. We will give no weight to the fact that the constancy of species seems by no means to be absolutely without exception; for on the whole, they certainly remain constant. The only example which goes to prove such an evolution of species as taking place to-day—viz: the natural history of sponges—seems not to have this bearing. The transitions of form, proven by O. Schmidt in the siliceous sponges and by Häckel in the chalk-sponges, seem to show, not the genetic coming forth of a new species out of another, and especially not the evolution of a higher species out of a lower, but rather the uncertainty of the idea of species in general and the worthlessness of the skeleton-forms, for this idea, in such low organizations as the sponges. But that objection already loses its chief force from the consideration that we have not only never observed the origin of one species from another, but never even the origin of a species itself; and that nevertheless all species have successively originated in time. If we, therefore, are not able to observe directly their origination, we have a right to make all possible attempts at approaching the knowledge of it in an indirect way. But we see this objection invalidated by still another