HISTORY OF ZOOLOGY. 41 



points in common that they can best be treated under a 

 common head. 



Cuvier and Carl E. v. Baer had taught that the sepa- 

 rate types of the animal kingdom are units, under each 

 one of which there lies a special structure and plan of de- 

 velopment peculiar to it ; farther, that there are no similari- 

 ties in structure and in the development forming a bridge 

 from type to type. The first of these two propositions is 

 still as formerly regarded as correct, but the second, which 

 alone is important for the Descent Theory, has become 

 quite untenable. All animals have in the cell a common 

 organic principle and are thereby brought close to one 

 another; all multicelrular animals agree during the first 

 stages of their development, while the fertilization, cleav- 

 age of the egg, and the formation of the first two germ- 

 layers agree in the principal important points, and vary 

 from one another only in such differences as occur within 

 one and the same type. Also the peculiarities which dis- 

 tinguish each type in structure and in the mode of develop- 

 ment do not occur in the animal world without interme- 

 diate phases. Especially from the branch of the worms 

 there lead off transitional forms to the other branches : 

 Balanoglossus to the Echinoderms, the Annelids and Peri- 

 pat us to the Arthropods, the Tunicates and Amphioxus to 

 the Vertebrates. In some representatives of each type the 

 structure and the mode of development are simpler, 

 thereby approaching to the conditions which obtain in the 

 other types. The existence of such transitional forms is 

 one of the most important proofs in favor of the Descent 

 Theory, and speaks against the assumption of a rigid un- 

 varying type in Cuvier's sense. 



Fundamental Law of Biogenesis. A fact that weighs 

 heavily in the balance in favor of the Descent Theory is 

 the fact that the structure and mode of development of 

 animals is ruled by a conformity to a law which at this 

 time can only be explained by the assumption of a com- 

 mon ancestry. Each animal during its development passes 

 through essentially the stages which remain permanent in 



