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increased. The progeny produced by the intercrossing of species, i.e. les hybrides, hybrids, is almost always barren; the progeny produced by the intercrossing of races, les metis, mongrels, is always fertile. If the hybrids are fertile in the first generation, the fertility always ceases after very few generations ; on the other hand, mongrels always remain fertile.”? Of course the fertility and barrenness of these crosses has only been ascertained by observations made in comparatively few species and races-in most cases the theory rests on conclusion from analogy. But it must be generally assumed, as the great physiologist Joh. Müller says, that, “The species of animals cannot possibly have been generated from one another. Judging by all that occurs now in the history of the animal world, they must have been created separately and independently of one another.” In other words, all the individuals of the same species might possibly be descended from one pair of ancestors. I have already said that this is not actually the case, but the individuals of different species cannot possibly have had common ancestors.
It would in no way derogate from the truth of this theory were it to be discovered that some animals which are classed as different species of the same genus in reality had a common ancestry; that, for instance, cats, lions, and tigers could be traced back to a common stock. In such a case we should have to correct the details of the zoological system, and whereas we now put down Felis domestica, Felis tigris, and Felis leo, etc., as species of the genus Felis, we should, according to the
1 Rapport, etc. p. 113. Cf. Th. Waitz, Anthropológie, i. 19-36. ? Physiologie, ii. 769.
above supposition, have to class them as races of the species Felis, just as bull-dogs, poodles, etc., are considered as races of the species Canis domestica. There would always be a limit to these alterations in our ideas of the species. No zoologist holding this theory would consent to consider the elephant, the mouse, the fly, and the whale as races of the same species, and as possible descendants from the same ancestor.
Now this theory, that species is lasting and immutable, is in our day met by the Theory of Descent, of which Darwin is the principal expositor. This theory asserts that all forms of plants and animals which exist, or ever have existed, can be traced back to a few ancestral forms, perhaps to one primordial form. What we now call species were formed in the same way as varieties and races have been formed; variety, species, genus, order, are simply ideal conceptions, which may be useful for the classification of animals and plants, but are genealogically quite unimportant. All living beings are related, in the sense that all may be traced back through countless individuals to a common stock; just as the numerous varieties, e.g. of doves, may be traced back to one common ancestral form, from which in the course of time they have developed.
1 Michelis, Der Organismus und die Kirche, Bern 1874, p. 18. He says, “The species may be quite rightly called a variety of the genus. In the genus the creative thought has reached an enduring form,” Cf. p. 12. “The bounds of variability are at any rate fixed by the idea of the genus. In determining the species, there is always more space for variation; no alteration is possible in the genus.” Even Linnæus was later on inclined to assume that the different species of a genus had originally only formed one species. Cf. V. Baer, Studien, p. 256. The Benedictine monk Augustine Calmet and others have supported similar views, in order to explain the housing of all the animals in the ark. Zöckler, Gesch. der Beziehungen, ii. 237.
Vixere fortes ante Agamemnona,-in a certain sense there were Darwinians even before Darwin, at least according to the title of de Quatrefages' book, there were “The Precursors of Darwin.” It is not without interest to look back on these, if only because Darwin's reputation will lose nothing by the retrospect; on the contrary, a comparison between his theory and that of his predecessors will show the grandeur and genius of his system.
As early as the last century Buffon (1707-1788) suggested that all the species of animals might perhaps be traced back to about forty original types; all the forms which we now call species had developed by degrees from these, not however, as Darwin supposes, by improvement, but, on the contrary, by degeneration; so that the present more imperfect animals are degenerate descendants of more perfect ancestors. Buffon's idea of the way in which this degeneration had taken place was somewhat naive. Bears, for instance, were obliged by certain unfavourable conditions to leave the land and take to swimming in the water; this naturally produced some effect on their bodily constitution, and by degrees they became seals. The seals gradually degenerated in the same way into dolphins and whales, and so on. Buffon himself afterwards retracted this opinion, and in his latter writings as in his earlier ones he maintains the usual theory of the immutability of species.
During Buffon's lifetime there appeared in Amsterdam in 1748 and 1756 an extraordinary French book,
1 Cf. Quatrefages, Charles Darwin et ses Précurseurs Francais. Paris 870.
entitled Telliamed, or Conversations between an Indian Philosopher and a French Missionary on the Diminution of the Sea. The author's name was Benoit de Maillet—the name is written backwards on the title-page,-and he died in 1738. His secretary, a certain Abbé Lemascrier, edited the work. Parts of the book are said to be very reasonable, but the sections in virtue of which he is called a precursor of Darwin certainly are not among them. I will only quote one passage, in which he explains how birds sprang from fishes. He thinks that if the process did not take place every day before our eyes, it would be much more difficult to believe that a caterpillar can turn into a butterfly than that a fish can turn into a bird. Flying-fish might perfectly easily have been driven at some time by the wind from the sea on to the land, where they fell into the bushes or on to the grass. There they found nourishment, but were not able to return to the sea. By the influence of the air their fins split, became fluffy, and turned into long feathers, the skin was covered with downy feathers, the lower fins became feet, the body was changed, the neck and beak grew longer, and behold, the fish had become a bird.'
Jean Lamarck (1744-1829) in his Zoological Philosophy, which was published in 1809, takes the matter more gravely. He thinks that only two primordial forms existed, the infusorian and the worm, both of which originated by spontaneous generation. From these two primordial forms all species of animals have
1 Tellamed, ou entretiens d'un philosophe indien, avec un missionaire français sur la diminution de la mer. Cf. Quatrefages, pp. 9, 19.
developed themselves gradually, on the whole progressing, but in a few cases retrogressing. From the infusorian spring the medusæ and their allies the molluscs, etc.; from the worm, the insects, fish, birds, mammals, and, of course, lastly man. In explaining the changes, Lamarck proceeds from the physiological fact, that use and exercise strengthens and developes the organs, whereas they are stunted by disuse. He assumes that if an animal is brought under new conditions, and is therefore obliged to conform itself to them, the efforts and exertions which it makes in consequence of this will ever develope new organs; while, on the other hand, the organs which it cannot use in the new conditions become stunted. For instance, a bird which is obliged to seek its food in the water, endeavours to move on the surface of the water, and for this purpose stretches out its toes. In consequence of the continued stretch- , ing out of the toes, the skin which unites them at their bases stretches, and at last becomes the membranous web. On the other hand, the strand snipe, which cannot swim, but only approaches the water in search of food, is constantly in danger of sinking into the mud. The bird, wishing to avoid this, stretches its legs as far as it can, and in consequence of this continual stretching of the legs of those birds, through many generations the legs become long and thin, as we see them in storks. One more example. A mollusc as it moves along tries to touch the bodies which lie before it. It endeavours to touch them with some of the more prominent parts of its head ; in consequence of these efforts the juices of its body flow with peculiar strength to these parts. By this means the nerves then are