I need not say to palæontologists that this is not what we actually find in the primordial rocks. I need but to remind them of the early and remarkable development of such forms as the Trilobites, the Lingulidæ and the Pteropods, all of them highly complex and specialized types, and remote from the embryonic stages of the groups to which they severally belong. In the case of the Trilobites, one need merely consider the beautiful symmetry of their parts, both transversely and longitudinally, their division into distinct regions, the necessary complexity of their muscular and nervous systems, their highly complex visual organs, the superficial ornamentation and microscopic structure of their crusts, their advanced position among Crustaceans, indicated by their strong affinity with the Arachnidans or spiders and scorpions. (See figures prefixed.) All these characters give them an aspect far from embryonic, while, as Barrande has pointed out, this advanced position of the group has its significance greatly strengthened by the fact that in early primordial times we have to deal not with one species, but with a vast and highly differentiated group, embracing forms of many and varied subordinate types. As we shall see, these and other early animals may be regarded as of generalized types, but not as embryonic. Here, then, meets us at the outset the fact that in as far as the great groups of annulose and molluscous animals are concerned, we can trace these back no farther than to a period in which they appear already highly advanced, much specialized and represented by many diverse forms. Either, therefore, these great groups came in on this high initial plane, or we have scarcely reached half way back in the life-history of our planet. We have, here, however, by this one consideration, attained at once to two great and dominant laws regulating the history of life. First, the law of continuity, whereby new forms come in successively, throughout geological time, though, as we shall see, with periods of greater or less frequency. Secondly, the law of specialization of types, whereby generalized forms are succeeded by those more special, and this probably connected with the growing specialization of the inorganic world. It is this second law which causes the parallelism between the history of successive species and that of the embryo.