162 THE MAIN CURRENTS OF ZOOLOGY 



remarkable clearness in Kellogg's Darwinism To-day, 

 to which volume the reader is referred for fuller 

 information. 



There are, however, two ideas of fundamental im- 

 portance in post-Darwinian discussions that should 

 receive mention. These are designated respectively, 

 orthogenesis and isolation. Theodor Eimer, since 

 1888, has been the typical representative of the ideas 

 of orthogenesis which means development in a 

 straight or definite direction. He maintains that 

 variations of organisms take place, not fortuitously 

 in radiating lines, but follow a few definite direc- 

 tions. He insists that variations are not preserved 

 on the basis of their utility, but as the result of the 

 direct inheritance of acquired characters. This is 

 intended as a replacing theory for that of natural 

 selection. 



Isolation as a favoring condition of species- 

 formation has been championed by Moritz Wagner 

 (since 1868) and, more recently, by David Starr 

 Jordan, Gulick, Romanes and others. This is based 

 on the obvious fact that slight variations will be 

 more likely to persist if the species in which they 

 occur are segregated, or isolated, so that those ex- 

 hibiting similar variations shall be compelled to 



