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 Zooelogy 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.

Cuvier. Baer.  Radiates, Peripheric, Mollusks, Massive, Articulates, Longitudinal, Vertebrates.  Doubly Symmetrical.

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,