CLASSIFICATION. 383 



another order of faets those of development. The embryo- 

 logical inquiries of Von Baer led him to arrange animals as 

 follows : 

 I. Peripherie Type. (RADIATA.) Evolutio radiata. The 



development proceeds from a centre, producing identical parts in a 

 radiating order. 



II. Massive Type. (MOLLUSCA.) Evolutio contorta. The 



development produces identical parts curved around a conical or other 

 space. 



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. (VEETEBRATA.) 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. 



Eecognizing 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 classi 

 fication of animals shows us the assumed linear order com 

 pletely broken up. In his lectures at the Eoyal 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 pub 

 lished 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 acci 

 dent of typographical convenience. Each of them is to be 

 regarded simply as a cluster. And if Prof. Huxley had fur 

 ther developed the arrangement, by dispersing the sub-groups 



