difficulties are again encountered in this connection. This 

 primitive form is a real bird according to Zittel; and ac- 

 cording to the same investigator as also according to 

 Marsh, Dames, Vetter, Parker, Tuerbringen, Parlow and 

 Mehnert, it is inadmissible to connect birds with a definite 

 class of reptiles. Haeckel finds his way out of the difficulty 

 by supplying hypothetical forms which no one has ever 

 seen, but which his imagination has admirably depicted as 

 transitional forms. In so doing, however, he abandons the 

 inductive method of natural science. 



It is impossible for us to treat at such length all the 

 remaining sections of this important book. We may men- 

 tion in passing that Fleischmann examines the "roots of 

 the mammal stock," and enters upon a detailed discussion 

 of "the origin of lung-breathing vertebrates," the "real 

 phylo-genetic problem of the mollusks," and "the origin of 

 the echinodermata." It is evident that he boldly takes up 

 the most important problems connected with the theory of 

 Descent, and does not confine himself to^ a one-sided dis- 

 cussion of individual points. As he did not fear to examine 

 thoroughly the famous, and as it hitherto appeared, invul- 

 nerable, "parade-horse," so neither does he hesitate to de- 

 molish the other reputed proof for the doctrine of Descent, 

 e. g., the fresh-water snail of Steinheim, the remains of 

 which Hilzendorf and Neumayr examined and were said 

 to have arranged in lines of descent that "would actually 

 stagger one." It is important to call especial attention to 

 this because the adversaries of the book ignore it. He next 

 shows up the so-called "fundamental principle of biogene- 



127 



