REVIEWS. 859 



pelling the faunas and floras to. migrate, or die, or by modifica- 

 tions to adapt themselves to their surroundings becomes a powerful 

 factor in the production of new varieties and the origin of new 

 species. 



Part II. takes up the mode of life of the marine fauna, especially 

 from the standpoint of their importance to the palaeontologic geologist. 

 In order that the working geologist may not underrate the sources of 

 error in his work the author very wisely discusses first the various 

 zoologic groups to show what ones are bv reason of the absence or 

 the unstable nature of the hard parts incapable of preservation. The 

 following sections discuss for the most part the conditions favorable to 

 the life of the various animals which are taken up in their zoologic 

 order beginning with the Foraminifera. In this section of the book 

 the tables showing the maximum and minimum depths at which vari- 

 ous species have been found by dredging are an especially prominent 

 and valuable feature. In the section on the Cephalopods especial 

 attention is called to the fact that the shells having air chambers by 

 being able to float for long periods may be readily driven to great dis- 

 tances after the death of the animal and become fossils in a totally 

 different region from that inhabited by the animal during life and in 

 varying sorts of rock according to the nature of the bottom upon 

 which the shell chances to drift. He points to the fact that the liv- 

 ing Spirula and Nautilus are restricted to narrow boundaries on the 

 sea bottom and that the animal is rarely seen. At the same time the 

 shells of the dead Nautilus and Spirula are found widely scattered on 

 tropical strands far from the home of the living animal. These thoughts 

 applied to the very important fossil group of the Ammonites mean that 

 their shells tell us nothing of the conditions under which the Ammonites 

 lived but give us most valuable means of making exact correlations ; 

 for if as we suppose the Ammonite shell on the death of the animal 

 rose to the surface and was carried for many days hither and thither 

 and finally filled and sank, the shells would be scattered here and 

 there over the entire ocean bottom and deposited in sediments all of 

 the same age. So confident is the author of this that he believes that 

 the Ammonite shells mark not only homotaxial but actually homocfonial 

 periods. 



Part III. considers the formation of rocks upon the present surface 

 of the earth. According to their mode of formation mechanical, chem- 

 ical, volcanic and organic deposits are distinguished, and the distribu- 



