The Principles of Biology, Volume 1 · Herbert Spencer
VERTEBRATA.
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. (_Abranchiata_) Mammalia Aves Reptilia (_Branchiata_) Amphibia Pisces. Cephalopoda Heteropoda } _Articulata._ Gasteropoda-dioecia } Insecta Arachnida } Myriapoda Crustacea { Pulmonata Gasteropoda-monoecia { Pteropoda _Annuloida._ Lamellibranchiata Annellata Scoleidæ Echinodermata Trematoda Rotifera Tæniadæ Turbellaria Nematoidea Hydrozoa Actinozoa.
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