III. Longitudinal Type. (Articulata.) Evolutio gemina. The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis.
IV. Doubly Symmetrical Type. (Vertebrata.) Evolutio bigemina. The development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures.
Recognizing these fundamental differences in the modes of development, as answering to fundamental divisions in the animal kingdom, Von Baer shows (among the Vertebrata at least) how the minor differences which arise at successively later embryonic stages, correspond with the minor divisions.
Like the modern classification of plants, the modern classification of animals shows us the assumed linear order completely broken up. In his lectures at the Royal Institution, in 1857, Prof. Huxley expressed the relations existing among the several great groups of the animal kingdom, by placing them at the ends of four or five radii, diverging from a centre. The diagram I cannot obtain; but in the published reports of his lectures at the School of Mines the groups were arranged as on the following page. What remnant there may seem to be of linear succession in some of the sub-groups contained in it, is merely an accident of typographical convenience. Each of them is to be regarded simply as a cluster. And if Prof. Huxley had further developed the arrangement, by dispersing the sub-groups and sub-sub-groups on the same principle, there would result an arrangement perhaps not much unlike that shown on the page succeeding this.
| VERTEBRATA |
| (Abranchiata) Mammalia Aves Reptilia (Branchiata) Amphibia Pisces. |
| MOLLUSCA | ANNULOSA | ||||
| Cephalopoda | Heteropoda |  | Articulata. | ||
| Gasteropoda- diœcia | Insecta | Arachnida | |||
| Myriapoda | Crustacea | ||||
 | Pulmonata Pteropoda | Gasteropoda- monœcia | Annuloida. | ||
| Lamellibranchiata | Annellata | Scoleidæ | |||
| Echinodermata | Trematoda | ||||
| Rotifera | Tæniadæ | ||||
| Turbellaria | |||||
| Nematoidea | |||||
| CŒLENTERATA | |
| Hydrozoa | Actinozoa. |
| PROTOZOA | ||
| Infusoria | Spongiadæ | Gregarinidæ |
| Noctilucidæ | Foraminifera | Thallassicollidæ |
In the woodcut, the dots represent orders, the names of which it is impracticable to insert. If it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an approximate idea will be formed of the relations among the successively-subordinate groups constituting the animal kingdom. Besides the subordination of groups and their general distribution, some other facts are indicated. By the distances of the great divisions from the general centre, are rudely symbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. Within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. And the distribution of the sub-groups within each group, is in most cases such that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them—in their analogies though not in their homologies. No such scheme, however, can give a correct conception. Even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface (which of course it does not), it would still be inadequate. Such relations cannot be represented in space of two dimensions, but only in space of three dimensions.
