60 GENERAL EVOLUTION. 



some of these developmental relations, that they are exhibited 

 by certain single structures only, and not by whole organisms. 

 These objectors must not forget that the distinctions of those 

 groujis, which alone we have in one geological period in a relation 

 of near affinity, exists in single characters only ; and that it is 

 therefore infinitely probable that the higher groups, when we 

 come to know their rejDresentatives with the same completeness, 

 will prove to be separated by single characters of difference also. 



3. The following table (pages 63 to 73) is introduced to illus- 

 trate the relations of groups higher than the preceding. This is 

 largely measured by the circulatory system, not only as to the 

 class relations, but also as regards orders. In its less central por- 

 tions it is, however, definitive of families at times.* (See also 

 Plate!) 



If the reader will comjoare the history of the development of 

 vertebrates of any class or order, as those of Teleosts and the 

 lizard by Lereboullet, of the snake and tortoise by Kathke and 

 Agassiz, and of the bird and mammal by von Baer, he will find 

 the most complete examples of the inexact paraTlelism of the 

 lower types with the embryonic stages of the higher. A few 

 points are selected as examples, from the histories included in a 

 few of the columns of the table, and given at its end. 



Similar parallels may be found to exist in the most beautiful 

 manner between the adult anatomv and structure of the uroo^eni- 



*/ CI? 



tal aj)paratus within each class of the series taken separately, as 

 indicating ordinal relationship. This department is, however, 

 omitted for the present. 



As an example of the homologies derivable from the circula- 

 tory system, and of the use of the following table, I give the fol- 

 lowing relations between the types of the origins of the aorta, f 



The single ventricle of Teleostei is no doubt homologous with 

 that of Lepidosteus, and that of Lepidosiren. The arteria vesiccB 



* This sketch is not nearly complete, but is published in hopes of its being use- 

 ful to students. It is compiled from the works of Meckel, Rathke, Barkow, Miiller, 

 Ilyrtl, Briicke, Stannius, and others, in connection with numerous dissections. 



f Prof. Agassiz (" Contrib. Nat. Hist. U. S.," i, p. 285) states that the ven- 

 tricle of the Testudinata " is not any more identical with the one ventricle of 

 fishes than with the two ventricles of warm-blooded vertebrata ; for in fishes we 

 find only one vessel, the aorta, arising from it, while in turtles both the ao7'ta and 

 arteria pulmonalis start to'^ether from it." We think this statement, which, if true, 

 is destructive to the asserted homologies of the circulatory system, can not be sub- 

 stantiated, for the reasons above given. 



