250 THE EVOLUTION OF MA2*. 



We must look for the original parent-form of the four 

 higher tribes of animals among the numerous branch-forms 

 of the Worm Tribe. The comparative Anatomy and 

 Ontogeny of these four tribes certainly teach that all origi- 

 nated from four different branches of Worms. This tribe is 

 the common ancestral group of the four higher animal tribes. 

 These last are : (1) the Star-animals (Echinoderma — Star- 

 fishes, Sea-urchins, Sea-lilies, Sea-cucumbers) ; (2) the im- 

 portant class of the Articulated-animals (Arthropoda — 

 Crabs, Spiders, Centipedes, Insects); (3) the Soft-bodied- 

 animals (Mollusca — Lamp-shells, Mussels, Snails, etc.) ; and 

 finally (4) the Vertebrata, the most highly developed tribe 

 of animals, to which Man belongs. 



These are the principles of the unified or monophyletic 

 genealogy of the animal kingdom, as they present them- 

 selves, provisionally, according to the Gastrsea Theory, at 

 the present stage of zoological classification and of embryo- 

 logical knowledge. If I am right in asserting the original 

 similarity or homology of the primitive intestine and the 

 two primary germ-layers enclosing it in all intestinal 

 animals, this phylogenetic classification of the animal 

 kingdom may supersede the systems hitherto based on the 

 Type Theory. According to this, therefore, the seven 

 types of that theory acquire a wholly different significance. 

 Of these seven tribes (Phyla), (1) that of the Protozoa 

 remains at the foot of the scale; from it springs (2) the 

 Gastrsea, which branches into the two lines of the Plant- 

 animals and Worms ; and from the Worms develop (3) the 

 four higher groups of animals ; these last are four diverging 

 lines, which are only connected together at the base, among 

 the lowest Worms, but are not otherwise comparable. 



