34: GENERAL PRINCIPLES OF ZOOLOGY. 



fortunately been found. Transitional forms connect the single- 

 toed horse of the present with the four-toed Eohippos of the 

 eocene; for all the hoofed animals a common starting-point or 

 ancestral form has been found in the Condylarthra. Transitional 

 forms have also been found between the greater divisions, as, e.g., 

 between reptiles and birds, the remarkable toothed birds, and the 

 Archceopteryx (fig. 2), a bird with a long, feathered, lizard-like 

 tail. 



(8) Morphological Proofs. When we employ comparative 

 anatomy and embryology in support of evolution, we find that the 

 two studies have so many points in common that they can best be 

 treated together. 



Cuvier and von Baer taught that the separate types of the 

 animal kingdom are units, each with a special structure and plan 

 of development 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 regarded 

 as correct, but the second, which alone is important for the theory 

 of evolution, has become quite untenable. All animals have a 

 common organic basis in the cell and are thereby brought close 

 to one another; all multicellular animals agree in the principal 

 points during the first stages of their development, during the 

 fertilization, cleavage of the egg, and the formation of the first 

 two germ-layers, and vary from one another only in such differ- 

 ences as may occur within one and the same type. Also the 

 peculiarities which distinguish each type in structure and in the 

 mode of development are not without intermediate phases. 

 Especially from the branch of the worms there lead off transitional 

 forms to the other branches : Balanoglossus to both echinoderms 

 and chordates, the annelids and Peripatus to the arthropods, the 

 tunicates and Amphioxus to the vertebrates. In some representa- 

 tives 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 theory of evolution, 

 and speaks against the assumption of a rigid unvarying type in 

 Cuvier's sense. 



Fundamental Law of Biogenesis. A fact that weighs heavily 

 in the balance in favor of the theory of evolution is the fact that 

 the structure and mode of development of animals is ruled by a 

 law which at present can only be explained by the assumption of 

 a common ancestry. Each animal during its development passes 



