CLASSIFICATION. 301 



Passing over sundry classifications in which the serial 

 arrangement dictated by the notion of ascending complexity, 

 is variously modified by the recognition of conspicuous 

 anatomical facts, we come to the classifications which recognize 

 another order of facts — those of development. The embryo- 

 logical inquiries of Yon Baer, led him to arrange animals as 

 follows : — 



I. Peripheric Type. (Rapiata.) 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. 



IY. Doubly Symmetrical type. (Yertebeata.) Evolutio 

 bigemina. The development produces identical 

 parts arising on both sides of an axis, growing up- 

 wards 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. 



Recognizing these fundamental differences in the modes of 

 evolution, as answering to fundamental divisions in the 

 animal kingdom, Yon Baer shows (among the Vertebrata at 

 least) how the minor differences that arise at successively 

 later stages of evolution, correspond with the minor divisions. 



Like the modern classification of plants, the classification 

 of animals that has now been arrived at, is one in wdiich the 

 linear order is completely broken up. In his lectures at the 

 Royal Institution, in 1857, Prof. Huxley expressed the rela- 



