Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/40

32 The Infusoria are not to be distinguished in their early stages from Amoebae. Kolpoda-Hke forms, supposed at one time to belong to the Infusoria, have since been shown to be the young of Turbellarian Worms. The Infusoria seem then to be essentially a transition group; so much so that some naturalists have held that the group is not a distinct division of the animal kingdom, but simply a collection of the young of higher animals. It seems proper to mention now the necessity of learning the condition of the young, or embryonic stage of animals, whose origin we are seeking. Supposing the animal kingdom is really represented by a tree, of which the main branches, twigs, and leaflets are the orders, families, and species into which animals are divided, common features of structure in these groups must not be sought at the ends of the branches which are far apart, but at the point where the branches diverge. To make my meaning clear, take the case of young babies, which look very much alike, but owing to certain hereditary influences, and the effects of a different mode of bringing up, can be readily distinguished later in life. The origin of Worms is not to be sought in comparing a highly-organized member of the group with one of the Infusoria, but by placing side by side a simple worm like the Planaria and one of the Animalcula. The proofs of the Worms coming from Infusoria are furnished by the resemblance of the young of the Soft Worms to existing Infusoria, and the peculiarities of structure common to both.

By looking at Tree III. we see the root of the Worms divides into two branches,—the Soft Worms (Scolecida) and Sac-worms,—the Soft Worms giving rise to the Articulated Worms, in which are seen the beginnings of the Echinodermata and Articulata, while the Sac-worms are the