EVOLUTIONAL ZOOLOGY 223 



attempts have been made on both geographical and physio- 

 logical grounds, but few naturalists have been convinced: that 

 the principle of isolation should be invoked as a factor of 

 evolution, however helpful segregation of any kind might be 

 in hastening the establishment of new species. 



Criticisms of the Second Group (Determinate Variation). 

 A second class of objections has been raised against natural 

 selection on the ground that the general fitness or adaptiveness 

 of organisms and the beginning of new characters could never 

 have arisen out of accidental and indiscriminate variations, 

 as was supposed by Darwin to have been the case. This rea- 

 soning has appealed with force to the minds of many who have 

 felt the necessity of assuming the existence of determinate 

 variation occurring in definite, beneficial lines in the produc- 

 tion of adaptations. If, then, it is maintained, there is some 

 underlying cause that calls forth the variation when needed 

 and directs its development toward a definite end, natural 

 selection must be secondary to this primary factor which con- 

 trols the beginning of variations essential to selective action. 

 The post-Darwinian period has witnessed the rise of an army 

 of theories based on the assumption of determinate variation. 

 They are all concerned with what has been termed Ortho- 

 genesis, or evolution along straight, definite and determinate 

 lines. Highly speculative and unverifiable as most of them 

 are, they are also premature, since it has been by no means 

 proved that evolution occurs in such definitely directed paths. 



The most conspicuous attempt to establish determinate 

 variation is to be found in the school of the Neo-Lamarckians 

 who have identified the cause of such variations with certain 

 of the old Lamarckian factors, especially that of the inherited 

 effects of the action of the environment. I cannot here do 

 more than merely refer to the interminable and fruitless dis- 

 cussion that has been carried on between the Neo-Lamarckians 

 and the Neo-Darwinians or the modern followers of Lamarck 

 and Darwin, respectively, over the question of the inheritance 



