414 THE GRAMMAR OF SCIENCE 



indeed be a valid argument against the Darwinian theory 

 of evolution. 



There is small doubt of the existence of a selective 

 death-rate. With a stable environment it is probably 

 largely periodic and not secular, though I am inclined to 

 think by no means absolutely so. To measure the 

 influence and rapidity of secular selection, we must change 

 environment, we must operate by transferring forms of 

 life to new localities, or by actual laboratory experiment. 

 As our planet is now in middle life, she has ceased, or 

 apparently ceased, from the vast inorganic experiments 

 she must have made in her youth. Such seems a reason- 

 able argument against those who assert that the secular 

 evolution induced by somewhat drastic laboratory ex- 

 periments has no bearing on natural selection in the past 



But suppose this first question answered, and let us 

 allow that secular selection is associated with change of 

 environment, how will this help us unless we can also 

 show that secular selection can produce differentiation ? 

 Our illustration, as indicated in the diagram on p. 409, 

 shows only the change in type produced by selection ; the 

 race as a whole changes, but we want to differentiate it, 

 to break it up into two types, and ultimately into two 

 groups relatively or absolutely infertile with each other, 

 i.e. into two species. Here new difficulties meet us, 

 which can only be conquered, not by hypotheses, but by 

 hypotheses studied under the light of statistics ; we want 

 numerical observation and quantitative experiment. We 

 wish to reach two groups relatively infertile. Now to 

 ascertain how this may be feasible we ought first to 

 answer the following problems : — 



{a) Are all members of a local race equally fertile 

 inter se ? If not, what is the degree of infertility, and is 

 it correlated with any combination of characters in the 

 two individuals concerned ? 



{b) To what extent are the reproductive organs and 

 the time and manner of their functioning correlated with 

 other organs and characters likely to be subject to selec- 

 tion under certain environments ? 



