26 GENERAL PRINCIPLES OF ZOOLOGY 



of these the mammals; among the mammals man. For smaller groups 

 genealogical material has 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 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 Archeeopteryx (fig. 2), a bird with 

 a long, feathered, lizard-like tail. 



(3) Morphological Proofs. When we employ comparative anatomy 

 and embryology in support of evolution, we find that the two have so 

 much 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 similarities in structure or develop- 

 ment forming a bridge from type to type. The first of these propositions 

 is still regarded as correct, but the second, which alone is important for 

 the theory of evolution, has become 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 from this time on only in such differences 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. 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 theory of evolution. 



Fundamental Law of Biogenesis. A fact that weighs heavily 

 in favor of the theory of evolution is that the structure and mode of develop- 

 ment 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 develop- 

 ment passes through essentially the stages which remain permanent in 

 the case of lower or better, more primitive animals of the same branch, 

 as the following examples show: (i) In the early stages of development 

 the human embryo (figs. 3, 609) possesses remarkable resemblances to 

 the lowest vertebrates, the fishes. Like these it has gill-slits, a simple 

 heart with auricle and ventricle; instead of a separation of aorta and 

 pulmonary arteries (body and lung arteries) a single arterial trunk going 

 from the heart; and aortic arches connecting this trunk with the descend- 

 ing aorta. All of these are structure adapted for branchiate respiration 



