PROGRESS 35 



to the stream of progress. By means of those which 

 march with that stream, the upper level of life's 

 attainment is raised. But the struggle still goes on : 

 and again, starting from this new condition, there will 

 be variations in every direction which will have 

 survival value, and some of these will be progressive ; 

 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 variation 

 occurring at one time in a living species is infinitesimal 

 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 mammal 

 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 respond- 

 ing brain machinery to be constructed in development, 

 and so advanced their possessors on to new shores 

 of control and independence. 



1 See Babcock and Clausen, ^i8. 



