xxi EVOLUTION OF EVOLUTION THEORIES 429 



of somatic modifications or individually acquired char- 

 acters directly due to peculiarities of nurture, has been 

 to strengthen the negative position, that the assumption 

 of the transmissibility of these changes in any direct 

 and specific way is not at present warranted by the facts. 

 As regards the process of selection, (1) one of the dis- 

 tinct steps of progress since Darwin's day has been the 

 demonstration of a few clear cases of natural selection 

 at work. That is to say, it has proved in a few cases 

 that the elimination is discriminate, that the survivors 

 survive in virtue of the presence of particular qualities. 

 Thus Cesnola tethered green Mantises on green herbage 

 and brown Mantises on brown herbage and found that 

 they escaped the eyes of birds. But green Mantises on 

 brown herbage and brow r n Mantises on green herbage 

 were rapidly picked off. (2) Another step has been a 

 recognition of the manifoldness of selective processes. 

 Thus there is " lethal ' selection, which works by the 

 discriminate elimination of the relatively less fit to the 

 given conditions, and " reproductive ' selection, which 

 works through the increased and more effective reproduc- 

 tivity incident on the success of the relatively more fit. 

 And again, there may be selection among the germ-cells 

 themselves, where extraordinary thinning of numbers 

 is prevalent ; and possibly an intra-germinal selection 

 among constituent items or determinants in the inherit- 

 ance. There may be selection of males by the females, 

 or of females by the males, or of the males by one another, 

 but little progress has been made since Darwin's day in 

 the study of preferential mating. (3) In the early days 

 of Darwinism there was sometimes a tendency, which 

 Darwin's works do not countenance, to think and speak 

 too simply of the processes of selection. It is clear that 

 the process need not in the least imply a sudden elimina- 

 tion of the relatively less fit, for a shortened life and a 

 less successful family will work equally well in the long 

 run. It must be realised that the complexity of the web 

 of life is such that even slight peculiarities may be 

 of critical moment in determining the survival of the 

 variants possessing them. And just as in sexual selec- 



