Darwin's theory soon found an enthusiastic corps of followers—on the continent, and especially in Germany, almost more than in his own country. The outlook into an entirely new explanation of the origin of man, and the probable use of this theory for attacks upon faith in a Creator and Master of the world, called wide-spread attention to it; and the theory opened to natural science itself entirely new impulses and paths, and promised the solution of many problems before which it had hitherto been compelled to stand in silence. To be sure, it threatened likewise to allure the mind from the slow but sure ways of solid study to the entertaining but insecure and aimless paths of imagination and hypothesis.
Among all the German followers of Darwin who adopted not only the idea of an origin of species through descent and evolution, but also the explanation of evolution by natural selection, and extended it so as to make the principle of selection of exclusive value, Ernst Häckel occupies the most prominent rank.
In his "General Morphology," published in 1866, and in his "Natural History of the Creation," the first edition of which appeared in 1868, and finally in his "Anthropogeny"[[2]] (why he does not say Anthropogony, we are nowhere informed), 1874, this scientist brought the new theory, which had been presented by Darwin in an almost bewildering flood of details, into connection and order, and, analyzing the powers active in natural selection, combined them into an entire system of laws. He
at once drew the origin of man also into the course of reasoning on the new theory, and sustained the theory by the discovery of the monera and other low organisms of one cell, as well as by special investigations of the calcareous sponges. For these labors, he was rewarded by the warm and unreserved acknowledgment which Darwin made to him in his work upon the origin of man, which was published subsequently to the "Natural History of the Creation." There Darwin says: "If this work had appeared before my essay had been written, I should probably never have completed it. Almost all the conclusions at which I have arrived, I find confirmed by this naturalist, whose knowledge on many points is much fuller than mine." Häckel's labors rendered still greater service to the Darwinian theory by dividing the organic world into three kingdoms: the protista kingdom, the vegetable kingdom, and the animal kingdom,—a division which solves in a most simple way the difficulty that was felt more and more of securing for the lowest organisms a place among the animals or plants. He further aided the theory by leaving the choice open to adopt either a uniform or multiform pedigree of the organisms and their kingdoms and classes, and by treating each class under both points of view; and finally, by fascinating experiments to bring before us in detail the hypothetical pedigrees of all classes of organisms from the protista kingdom up to man.
We will try to reproduce briefly the pedigree which is of most interest—the hypothetical pedigree of man. Häckel divides it into twenty-two stages, eight of them belonging to the series of the invertebrates, and fourteen to that of the vertebrates. On this ladder of
twenty-two rounds, he leads us from the lowest form of the living being, in slight and mostly plausible transitions, continually higher and higher, up to man; and makes our steps easy by mentioning at each stage, on the one hand the corresponding state in the embryonic development, on the other the still living creature through which, in his opinion, the former organisms of the corresponding round of the ladder are still represented, and which accordingly has been a creature that remained on its round, while other members of its family have been developed up to man and to many other genera and species.
He begins with the monera, the organisms of the lowest form, discovered by himself, which have not so much as the organic rank of a cell, but are only corpuscules of mucus, without kernel or external covering, called by him cytod, and arising from an organic carbon formation. The lowest and most formless moneron is the bathybius, discovered by Thomas Huxley, a network of recticular mucus, which in the greatest depths of the sea, as far down as 7,000 metres, covers stone fragments and other objects, but are also found in less depths, in the Mediterranean Sea, for instance. From the moneron he proceeds to the amœba—a simple cell, with a kernel, which still corresponds to the egg of man in its first state. The third stage is formed by the communities of amœbæ (synamœbæ), corresponding to the mulberry-yolk in the first development of the fecundated egg, and to some still living heaps of amœbæ. To the fourth stage he assigns the planæa, corresponding to the embryonic development of an albumen and the planula or ciliated
larva. When these ciliated larvæ are developed, they contract themselves so as to form a cavity; and this fifth stage—especially important for his theory—he calls gastræa. In this form, he says, the progaster is already developed, and its wall is differentiated for the first time into an animal or dermal layer (ectoblast), and into a vegetative or intestinal layer (hypoblast). At the sixth stage, there branched off the prothelmis, or worms, with the first formations of a nervous system, the simplest organs of sense, the simplest organs for secretion (kidneys) and generation (sexual organs), represented to-day by the gliding worms or turbellaria; as the seventh stage, the soft worms, as he called them at first—the blood worms, or cœlomati, as he describes them in his "Anthropogeny"—a purely hypothetical stage, on which a true body-cavity and blood were formed; the eighth stage are the chorda-animals with the beginning of a spinal rod, corresponding to the larva of the ascidiæ. At the ninth stage, called the skull-less animals (acrania), and corresponding to the still living lancelet, we enter the series of the vertebrates. The importance of the eighth and ninth stages for the theory, we have already pointed out in our remarks upon Darwin, [p. 43]. The tenth stage is formed by those low fishes in which the spinal rod is differentiated into the skull—and the vertebral-column, called the single-nostriled animals (monorrhini), and represented by the cyclostoma of today (hag and lampreys). The eleventh stage is formed by the primæval fish or selachii (sharks); the twelfth by the mud fish, of which there still live the protopterus in Africa, the lepidosiren in the tributaries of the Amazon, and the ceratodus in the swamps of Southern
Australia. On the thirteenth stage, there are the gilled amphibians (sozobranchia), proteus and axolotl; on the fourteenth, the tailed amphibians (sozura), newt and salamander; on the fifteenth, the purely hypothetical primæval amniota or protamnia (amnion is the name given to the chorion which surrounds the germ-water and embryo of the three higher classes of vertebrates) on the sixteenth, the primary mammals (promammalia), to which the present monotremes (ornithorhynchus and echidna) stand nearest; on the seventeenth, the pouched animals or marsupialia; on the eighteenth, the semi-apes or prosimiæ (loris and maki); on the nineteenth, the tailed apes, or menocerca (nose-apes and slender-apes, or semnopithecus); on the twentieth, the man-like apes (anthropoides) or tail-less catarrhini (gorilla, chimpanzee, orang outang and gibbon). And now we come to twenty-one—ape-like men or speechless primæval men (alali)—of whom we are reminded to-day by the deaf, and dumb, the cretins and the microcephali; and number twenty-two is homo sapiens, the man. The Australians and the Papuans are supposed to be the only remaining representatives of his first stage-development. In like manner, Häckel also gives us the stem-branches of all the types, classes and orders of the organisms, and forms from them a very acceptable hypothetical pedigree; or—if we prefer to suppose a polyphyletic rather than a monophyletic origin of species—hypothetical pedigrees of the whole organic world.
The perspicuity and clearness of Häckel's deductions, the extent of his knowledge, and the singleness of his aim, to which he makes them all subservient, lend