24^0 ERNST HAECKEL. 



also Baer, took each type for something absolutely persistent, 

 and, in spite of all modifications, in the deepest sense un- 

 alterable ; consequently here they allowed no connection of 

 any sort and no transition between the four different types, 

 Baer, besides^ insists that the type of the lowest forms of 

 each of the four groups must be pronounced as well defined 

 as in the highest, and that, consequently, the type of develop- 

 ment is entirely independent of the grade of improvement. 



In contrast with the earlier prevailing erroneous opinion 

 that the Avhole of the animal kingdom represented a single un- 

 interrupted gradual scale of beings, and that a single con- 

 tinuous succession of development proceeded from the lowest 

 of the Infusoria through the different classes up to Man him- 

 self, the light which the type-theory threw over the different 

 portions of zoology, but particularly over comparative 

 anatomy and over the history of development, prociired for 

 it a speedy entrance into the zoological system, and the four 

 types were soon pretty commonly looked upon as the basis 

 of very exact scientific system of animals. One was, in- 

 deed, soon compelled, through the advance in one's know- 

 ledge of the lower animals, to pull to pieces that very un- 

 natural type Radiata. First, Siebold in 1845 separated from 

 it the Protozoa, and at the same time he divided the Articu- 

 lata into Arthropoda and Vermes. Leuckart, in 1848, was 

 the first to distinguish as two distinct types the Ccelenterata 

 and Echinodermata. So, from the original four types arose 

 the seven diverse main groups, which to this day are still 

 also in vogue in most systems equally as the highest and 

 most general of the chief divisions of the animal kingdom. 

 Rut the peculiar essence and the original signification of the 

 theory of types was not touched by the augmentation of the 

 number of types. The aims of the later zoologists was much 

 more directed to defining by the same standard the four new 

 types (Protozoa, Ccelenterata, Echinodermata, and Vermes), 

 and combining each of these as an isolated form entity, with 

 the peculiar "plan of structure," in which was the basis of 

 arrangement for the three retained older types (Arthropoda, 

 MoUusca, Vertebrataj of Baer and Cuvier. The idea, ever 

 since then, growing stronger of the entirely independent cha- 

 racter and the immanent "structure plan" of these seven 

 types of animals is to this day still generally prevalent ; so 

 that, for example, Claus, even in the newest edition of his 

 'Zoology' (1872, p. 41), points out the type- theory as the 

 most important advance in science since Aristotle, and as 

 the very foundation of the natural symptom systems - Even 

 Hopkins names the types, moreover, "the Kepler's laws 



