114 ORGANIC EVOLUTION— THK FACTORS 



instance, among men, the great differences frequently 

 observable among the children of the same parents, 

 cannot be due to the parents having acquired and 

 transmitted different traits at different periods of their 

 lives, for if among the offspring of dogs or fowls there 

 are great differences due to inborn variations alone, so 

 also must there be among the offspring of men. Do 

 we not all know families, some members of which are 

 bigger and stronger than the others ? In them we see 

 a co-adaptive variation of all the structures, and if 

 among men strength were important as a factor in 

 survival, we should by the survival of the strong 

 individuals get an evolution in strength. Do we not 

 all know families, some members of which can run 

 more swiftly than the others ? In them we see a 

 co-adaptive variation of all the structures which sub- 

 serve speed ; and if among men speed were important 

 as a factor in survival, we should by the survival of the 

 swiftest get an evolution in speed. Do we not all know 

 families, some of the members of which are taller than 

 the others ? In them we see a co-adaptive variation of 

 many structures ; and if among men height were im- 

 portant as a factor in survival, we should by the con- 

 tinued survival of the tallest get an evolution in height. 

 It is needless to multiply examples. Nothing can be 

 more certain than that which is denied by Mr. Spencer, 

 viz. that co-operative parts vary together. 



But how shall -we surmount the theoretical objection 

 made by him, viz. that it is highly improbable that a 

 thousand co-operative parts should vary, not only 

 together, but proportionately as well ? We shall sur- 

 mount it by taking cognizance of that which has been 

 so strangely overlooked by biologists, viz. that the 

 action of Natural Selection has not only devfeloped 

 complex and heterogeneous structures in the higher 

 animals whereby they are brought into greater har^ 



