THEORIES OF WEISMANN AND hi. \l:ll 413 



receive brief consideration here. These arc designated re- 

 spectively, orthogenesis and isolation. Theodore Earner is 

 the typical representative of the ideas of orthogenesis. He 



maintains that variations of organisms take place not for- 

 tuitously in radiating and heterogenous lines, but follow a 

 few definite directions. This definitely directed evolution 

 is called orthogenesis. He insists that there is continuous 

 inheritance of acquired characters, and he is radically op- 

 posed to the belief that natural selection plays an important 

 part in evolution. Variations are not preserved on the basis 

 of their utility, but as the result of the direct inheritance of 

 acquired characters. His theory was launched in 1888 (Or- 

 ganic Eovlution, 1889) and, as developed by Eimer, is to be 

 classed as a replacing theory. The title of his translated 

 pamphlet, published in English in 1898, On Orthogenesis and 

 the Impotence of Natural Selection in Species-Formation^ is 

 suggestive as to his position in reference to natural selection. 

 Isolation as a favoring (or even indispensable) condition 

 of species-formation has been championed by Moritz Wagner 

 (since 1868), by David Starr Jordan, Gulick, Romanes, and 

 others. This is based on the assumption that isolation of 

 species has played an essential part in the perpetuation of 

 variations. Isolation is assumed to act upon variations after 

 they are started and not to play an important part in pro- 

 ducing variations. The basal question is, Under what condi- 

 tions will variations persist and become intensified? If free 

 intercrossings occur, it seems likely that variations, which 

 at the beginning are slight, will tend to disappear. Accord- 

 ingly, it will be advantageous to have species living under 

 such conditions of segregation that those possessing similar 

 variations shall be compelled to breed together. This would 

 be accomplished by isolation of species either by geographical 

 barriers or by physiological infertility among two sections 

 of a species occupying the same territory. Romanes, who so 



