402 The Inward Urge to Change 



reaction of living matter to external physical and chemical 

 conditions. He attempted to account for adaptation and 

 orthogenetic evolution as due to internal properties of living 

 matter, specifically to the " laws of growth." He acknowl- 

 edged the effects of natural selection in extreme cases. He be- 

 lieved, however, that the outcome of the evolutionary process 

 would have been substantially the same if there had not been a 

 severe struggle for existence. The constitution of species is 



U 



I 



> Eyeless 



Fig. ^^. Bar-eye in Fruit Flies 



Among fruit flies used in the experimental study of heredity 

 there have appeared mutations in which the eye surface is re- 

 duced to a mere " bar "; and there have also been mutations 

 showing various degrees of reduction. A series of flies can be 

 arranged showing a continuous gradation from the smallest to 

 the largest eye. From actual observation, however, it is found 

 that each of the various forms can arise from the normal wild 

 type, and also that there can be reversion to the round eye. 

 The arrows indicate sources and directions of observed muta- 

 tions. After Zeleny. 



such, Eimer thought, that determinate variation and de- 

 terminate evolution ensue directly. 



The paleontologists have had the best opportunity to 

 observe the long-time facts regarding plants and animals. 

 They are disposed more than any other group to think of 

 evolution as determinate. Orthogenesis almost forces itself 

 upon the thought of one who sees the long succession of 

 snails, the coiling of ammonites and nautilus through the 

 ages, the increase in size of reptiles, the progressive elabora- 

 tion of tusks among the elephants, the steady overgrowth 



