234 GEOLOGICAL BIOLOGY. 



plans of structure upon which the multitudes of animals were 

 built, does not carry with it any theory as to the reasons for 

 the differences, or as to the mode by which the several types 

 of structure came to be carried out in such multitudinous 

 fashion. 



The fact is beyond dispute that there are a few types of 

 construction to which the animals of the whole kingdom con- 

 form, and these are expressed in the mature forms as well as 

 in the course of the individual development. Cuvier, when 

 lie considered the mature results, found them to be four; no 

 one since has found reason to dispute the validity of three of 

 them. The branches Ccelenterataand Echinodermata, consti- 

 tute one of them the " Animalia Radiata;" the Arthropoda 

 typify another the " Animalia Articulata;" and the Verte- 

 brates include substantially the same organisms classified by 

 Cuvier under the type " Animalia Vertebrata." 



Von Baer's Embryological Classification. Von Baer, from a 

 study of the course of embryologic growth, thought he had 

 found a more intrinsic reason for the types in the modes of 

 their embryonic growth than in the gross result. He defined 

 them as the " peripheric type," with evolutis radiata (i.e., 

 the Radiata); 2d, the (( massive type " (Mollusca), with " evo- 

 lutis contorta;" 3d, the "longitudinal type" (Articulata), 

 with evolutis gemina, or production of symmetrical parts on 

 both sides of an axis; 4th, " doubly symmetrical type " (Ver- 

 brata), "with evolutis bigemina" i.e., the development pro- 

 ducing symmetrical development on both sides of a median 

 axis, and also developing two cavities, one above and one 

 below the central axis. 



Fundamental Divisions of Classification discerned by Earlier 

 ^Naturalists. If we throw the light of more recent investiga- 

 tions upon the matter, we find it necessary to make expan- 

 sion of those divisions; but very little alteration in the funda- 

 mental classification, thus early recognized, is required in 

 order to express the scientific classification of present usage. 

 Looking upon classification from this point of view, we first 

 divide the Animal Kingdom on the fundamental character 

 of cell-growth. When cell-growth proceeds no higher than 

 the multiplication of cells, having no differentiated and 



