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.
III
From the time that Linnæus showed us the necessity of a scientific system as a framework for the arrangement of scientific facts in Natural History, the number of divisions adopted by zoölogists and botanists increased steadily. Not only were families, orders, and classes added to genera and species, but these were further multiplied by subdivisions of the different groups. But as the number of divisions increased, they lost in precise meaning, and it became more and more doubtful how far they were true to Nature. Moreover, these divisions were not taken in the same sense by all naturalists: what were called families by some were called orders by others, while the orders of some were the classes of others, till it began to be doubted whether these scientific systems had any foundation in Nature,