50O THE GRAMMAR OF SCIENCE 



the selective death-rate in man has a most substantial 

 value. No reader who has appreciated these figures can 

 possibly agree with the statement with regard to natural 

 selection made by Lord Salisbury in 1894,^ that " no man, 

 so far as we know, has seen it at work." It is at work, 

 and at work among civilised men, where intra-group 

 struggle, i.e. autogeneric selection, is largely suspended, 

 with an intensity of a most substantial kind. Of the 

 existence of natural selection there can be no doubt, 

 we require careful experiments and observation to indicate 

 the rapidity of its action. In a few years we may hope no 

 longer to hear natural selection spoken of as hypothetical, 

 but rather to listen to a statement of its quantitative 

 measure for various organisms under divers environments. 



'fc>' 



S 15. — Co7icluding Reinarks 



The reader who has followed the author through the 

 somewhat difficult quantitative discussions of this and the 

 previous chapters, will probably arise from the perusal 

 with the conviction that biology is almost as exact as any 

 branch of physical science. Our knowledge of atoms and 

 our application of atomic and molecular hypotheses to 

 problems in heat, elasticity, and cohesion is essentially 

 based on statistics of average conduct. Corpuscles in 

 each other's presence are supposed to obey certain laws of 

 motion, but no explanation has hitherto been given of 

 these laws. So it is with vital units ; they vary, why 

 they vary we know not, and we explain nothing by 

 attributing it to bathmic influences.^ As we can predict 

 little or nothing of the individual atom, so we can predict 

 little or nothing of the individual vital unit. We can 

 deal only with statistics of average conduct. We have 

 laws of variation and laws of heredity, in themselves quite 

 as general and as definite as the majority of those we 

 meet with in physics. The object of the naturalist is in 

 both cases the same, i.e. to replace^ the longer and more 



1 Presidential Address ; British Association, Oxford, 1894. 



2 Batlimic is only a convenient word to distinguish inherent from environ- 

 mental influences. 



