PROGRESS, BIOLOGICAL AND OTHER 35 



and SO the upper level will be once more raised. 



The process will take time, for, whatever theory of 

 variation we may hold ^ — the old idea of small con- 

 tinuous variations; or that of large mutations big 

 enough to produce new species at one jump; or the 

 most probable theory of numerous small mutations 

 — they one and all must grant that the largest varia- 

 tion occurring at one time in a living species is in- 

 finitesimal in comparison with the secular changes of 

 evolution. 



There will further be a premium upon progressive 

 changes, since a progressive change will generally 

 land its possessor in virgin soil, so to speak; if not 

 in an actually new physical environment, then in 

 a biologically new situation. The placental mam- 

 mal occupies the same dry land as did the wonderful 

 reptilian types of the Secondary epoch. But con- 

 stant temperature and embryonic nutrition within 

 its mother provide delicately adjusted conditions in 

 the early phases of development which in their turn 

 enabled a more elaborate and more delicately re- 

 sponding brain machinery to be constructed in de- 

 velopment, and so advanced their possessors on to 

 new shores of control and independence. 



There will thus be a constant biological pressure 

 (to use a term which, though still symbolic, a mere 

 analogy, is less misleading and question-begging than 

 elan vital) tending to push some of life on to new 

 levels of attainment, new steps in progress, because 



,?5ee Babcock and Clausen, '19. 



