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 Linnaeus 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 zooelogists 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, or signified anything more than that it had pleased Linnaeus, for instance, to call certain groups of animals by one name, while Cuvier had chosen to call them by another.