one division, as carnivorous animals, and the other three in another division as herbivorous animals,—and looking a little farther, we perceive, that, in common with the Cow and the Deer, the Goat and the Sheep have cloven feet, and that they are all ruminants, while the Horse has a single hoof, does not ruminate, and must therefore be separated from them, even though, like them, he is herbivorous.
This is but the simplest illustration, taken from the most familiar objects, of this comparative method; but the same process is equally applicable to the most intricate problems in animal structures, and will give us the clue to all true affinities between animals. The education of a naturalist, now, consists chiefly in learning how to compare. If he have any power of generalization, when he has collected his facts, this habit of mental comparison will lead him up to principles, to the great laws of combination. It must not discourage us, that the process is a slow and laborious one, and the results of one lifetime after all very small. It might seem invidious, were I to show here how small is the sum total of the work accomplished even by the great exceptional men, whose names are known throughout the civilized world. But I may at least be permitted to speak of my own efforts, and to sum up in the fewest words the result of my life’s work. I have devoted my whole life to the study of Nature, and yet a single sentence may express all that I have done. I have shown that there is a correspondence between the succession of Fishes in geological times and the different stages of their growth in the egg,—this is all. It chanced to be a result that was found to apply to other groups and has led to other conclusions of a like nature. But, such as it is, it has been reached by this system of comparison, which, though I speak of it now in its application to the study of Natural History, is equally important in every other branch of knowledge. By the same process the most mature results of scientific research in Philology, in Ethnology, and in Physical Science are reached. And let me say that the community should foster the purely intellectual efforts of scientific men as carefully as they do their elementary schools and their practical institutions, generally considered so much more useful and important to the public. For from what other source shall we derive the higher results that are gradually woven into the practical resources of our life, except from the researches of those very men who study science not for its uses, but for its truth? It is this that gives it its noblest interest: it must be for truth’s sake, and not even for the sake of its usefulness to humanity, that the scientific man studies Nature. The application of science to the useful arts requires other abilities, other qualities, other tools than his; and therefore I say that the man of science who follows his studies into their practical application is false to his calling. The practical man stands ever ready to take up the work where the scientific man leaves it, and to adapt it to the material wants and uses of daily life.
The publication of Cuvier’s proposition, that the animal kingdom is built on four plans, created an extraordinary excitement throughout the scientific world. All naturalists proceeded to test it, and many soon recognized in it a great scientific truth,—while others, who thought more of making themselves prominent than of advancing science, proposed poor amendments, that were sure to be rejected on farther investigation. There were, however, some of these criticisms and additions that were truly improvements, and touched upon points overlooked by Cuvier. Blainville, especially, took up the element of form among animals,—whether divided on two sides, whether radiated, whether irregular, etc. He, however, made the mistake of giving very elaborate names to animals already known under simpler ones. Why, for instance, call all animals with parts radiating in every direction Actinomorpha or Actinozoaria, when they had received the significant name of Radiates? It seemed, to be a new system, when in fact it was only a new name. Ehrenberg, likewise, made an important distinction, when he united the animals according to the difference in their nervous systems; but he also incumbered the nomenclature unnecessarily, when he added to the names Anaima and Enaima of Aristotle those of Myeloneura and Ganglioneura.
But it is not my object to give all the classifications of different authors here, and I will therefore pass over many noted ones, as those of Burmeister, Milne, Edwards, Siebold and Stannius, Owen, Leuckart, Vogt, Van Beneden, and others, and proceed to give some account of one investigator who did as much for the progress of Zoölogy as Cuvier, though he is comparatively little known among us. Karl Ernst von Baer proposed a classification based, like Cuvier’s, upon plan; but he recognized what Cuvier failed to perceive,—namely, the importance of distinguishing between type (by which he means exactly what Cuvier means by plan) and complication of structure,—in other words, between plan and the execution of the plan. He recognized four types, which correspond exactly to Cuvier’s four plans, though he calls them by different names. Let us compare them.
Though perhaps less felicitous, the names of Baer express the same ideas as those of Cuvier. By the Peripheric he signified those in which all the parts converge from the periphery or circumference of the animal to its centre. Cuvier only reverses this definition in his name of Radiates, signifying the animals in which all parts radiate from the centre to the circumference. By Massive, Baer indicated those animals in which the structure is soft and concentrated, without a very distinct individualization of parts,—exactly the animals included by Cuvier under his name of Mollusks, or soft-bodied animals. In his selection of the epithet Longitudinal, Baer was less fortunate; for all animals have a longitudinal diameter, and this word was not, therefore, sufficiently special. Yet his Longitudinal type answers exactly to Cuvier’s Articulates,—animals in which all parts are arranged in a succession of articulated joints along a longitudinal axis. Cuvier has expressed this jointed structure in the name Articulates; whereas Baer, in his name of Longitudinal, referred only to the arrangement of joints in longitudinal succession, in a continuous string, as it were, one after another. For the Doubly Symmetrical type his name is the better of the two; for Cuvier’s name of Vertebrates alludes only to the backbone,—while Baer, who is an embryologist, signifies in his their mode of growth also. He knew what Cuvier did not know, that in its first formation the germ of the Vertebrate divides in two folds: one turning up above the backbone, to inclose all the sensitive Organs,—the spinal marrow, the organs of sense, all those organs by which life is expressed; the other turning down below the backbone, and inclosing all those organs by which life is maintained,—the organs of digestion, of respiration, of circulation, of reproduction, etc. So there is in this type not only an equal division of parts on either side, but also a division above and below, making thus a double symmetry in the plan, expressed by Baer in the name he gave it. Baer was perfectly original in his conception of these four types, for his paper was published in the very same year with that of Cuvier. But even in Germany, his native land, his ideas were not fully appreciated: strange that it should be so,—for, had his countrymen recognized his genius, they might have claimed him as the compeer of the great French naturalist.
Baer also founded the science of Embryology, under the guidance of his teacher, Dollinger. His researches in this direction showed him that animals were not only built on four plans, but that they grew according to four modes of development. The Vertebrate arises from the egg differently from the Articulate,—the Articulate differently from the Mollusk,—the Mollusk differently from the Radiate. Cuvier only showed us the four plans as they exist in the adult; Baer went a step farther, and showed us the four plans in the process of formation. But his greatest scientific achievement is perhaps the discovery that all animals originate in eggs, and that all these eggs are at first identical in substance and structure. The wonderful and untiring research condensed into this simple statement, that all animals arise from eggs and that all those eggs are identical in the beginning, may well excite our admiration. This egg consists of an outer envelope, the vitelline membrane, containing a fluid more or less dense, the yolk; within this is a second envelope, the so-called germinative vesicle, containing a somewhat different and more transparent fluid, and in the fluid of this second envelope float one or more so-called germinative specks. At this stage of their growth all eggs are microsopically small, yet each one has such tenacity of its individual principle of life that no egg was ever known to swerve from the pattern of the parent animal that gave it birth.
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