NATURAL SELECTION 311 



Whether this is due to the better regulated internal relations 

 of such individuals or to their adaptation to the mean conditions 

 of their habitat is still quite unknown. The few instances of 

 historical changes in natural populations which we have been 

 able to collect throw little light on the causes of the changes. 

 Even in the melanic Lepidoptera the elimination of lighter 

 individuals on a darkened background has not been the 

 subject of a detailed investigation. 



The direct evidence for the Natural Selection theory would 

 carry little conviction without the support of much indirect 

 evidence, but we have emphasised the necessary limitations 

 of the latter, which consists, essentially, in demonstrating that 

 organisms are more or less adapted to their environment. 

 Now some fundamental properties of living organisms, such as 

 irritability or cellular respiration, are definitely adaptive and 

 yet can hardly be regarded as the result of selection, since 

 without them we cannot imagine a living organism existing. 

 Adaptation is therefore to some extent synonymous with life, 

 and an extended series of adaptive relationships does not 

 necessarily tell us very much as to how these relationships 

 arose. The theory that Natural Selection has produced all 

 such relationships is attractive, because there is no other 

 widely applicable theory in the field ; but the proof of the 

 Natural Selection theory depends, in the last resort, on obser- 

 vations of death-rates, not on descriptions of the adaptations 

 of the living. 



Under our third heading we have considered some of the 

 genetical data as to the nature of variation and have endeavoured 

 to decide whether the material provided is at all suitable for 

 the efficient operation of a selective process. We have also 

 criticised the purely deductive evolutionary theories which 

 have been founded almost entirely on the mathematical 

 treatment of genetical data. Our knowledge of mutation 

 under laboratory conditions might be summarised by saying 

 that mutants are relatively rare and mostly harmful. It is 

 possible that beneficial mutants also occur, but this is still 

 largely an assumption, though perhaps a somewhat credible one. 

 We have no data which allow us to assume an approximate 

 mutation-rate for most species, and, for the few in which 

 some evidence is available, it is scarcely certain that under 

 natural conditions the rate would be the same. Even if it is 



