GEOGRAPHICAL DISTRIBUTION. I2/ 



school, on the one hand, holds that environment is the chief 

 factor determining the direction and extent of the modifica- 

 tions, which heredity tends to perpetuate, and that ancestry 

 plays only the part of holding and preserving, in its offspring, 

 what it gets from the agency of environment. 



The other extreme is the opinion that ancestry is the more 

 efficient factor in bringing about the evolution ; that in what 

 is called variability there is working out, not a mere acci- 

 dental reflex of environment upon the plastic organism, but a 

 fundamental property or force of organisms, ever tending 

 from homogeneity to heterogeneity, and resulting in the 

 specialization of functions and the differentiation of organic 

 structure always ; the line of evolution followed out by any 

 particular race being influenced little by environment, the 

 adjustments being active and not passive, the successful 

 organisms seeking and adopting conditions favorable for their 

 existence if out of them, dying out if the conditions favor- 

 able are not within reach, or if crowded out of them. 

 Natural selection, to this school of opinion, plays rather an 

 eliminating role than one of causation, and explains rather 

 why there are gaps in the series of organisms than why the 

 characters assumed in the modified forms are what they are. 

 In this latter view the successive steps of modification of a 

 race are as much controlled by the ancestry as are the succes- 

 sive steps of development in the growth of the individual. 



In the former view there is the replacement of the theory 

 of immutability of species by that of the mutability of species, 

 but the process of reproduction is still looked upon as immut- 

 able, reproducing the characters of the parents in the offspring 

 without change ; in the second view reproduction itself takes 

 a part in evolution and normally accomplishes modification of 

 form, either slowly or suddenly, but progressively, and evolu- 

 tion is an intrinsic law of, organism. 



An Unknown Cause assumed to explain Origins by both Forbes 

 and Lamarck. The naturalists of Forbes' school, with the 

 fundamental notion of immutability of species, had no other 

 way to explain the series of successive forms which they knew 

 from paleontological research than to call in the resources of 

 a first cause; but they were not ignorant of the series. 



