No. r>64] 



THE FIXATION OF CHARACTER 



729 



4. The principal categories of conservative characters 

 are those of number, position and plan. 



5. Particular organs or regions of the body, throughout 

 large groups of animals and plants, are less subject to 

 change than others and hence are seats of primitive 

 characters. 



6. The early ontogenetic stages of animals and plants 

 repeat those characters which were most conservative and 

 firmly fixed in their ancestry. 



7. Evolutionary advance and increase in differentia- 

 tion tend to result in the decrease of variability. This 

 is analogous to the loss of potentialities during ontogeny 

 and is also comparable to the formation of habit. 



8. Organic evolution is dependent on the action of two 

 opposing factors : that of progressive fixation, which tends 

 universally toward greater rigidity and conservatism in 

 all characters during evolutionary advance; and that of 

 natural selection, which tends to maintain or increase the 

 variability of those characters important for survival by 

 eliminating individuals where such characters have be- 

 come so fixed that the organism fails to possess a neces- 

 sary degree of adaptability. Natural selection is not con- 

 cerned with harmless and trivial characters which conse- 

 quently tend to become very conservative and are of much 

 value in classification. 



9. Such general principles of phylogeny as these, if 

 thoroughly established and defined, will make possible 

 the construction of a truly natural classification of organ- 

 isms on a logical and uniform basis. They also present 

 a clearer conception of the general method of evolution 

 than is set forth by the theory of natural selection alone. 



The writer is much indebted to Professor Herbert W. 

 Rand, of Harvard University, for suggestions and infor- 

 mation. 



