1 20 Evolution and Adaptation 



must be annually destroyed by accidental causes, which would 

 not be in the least degree mitigated by certain changes of 

 structure or constitution which would in other ways be bene- 

 ficial to the species. But let the destruction of the adults 

 be ever so heavy, if the number which can exist in any dis- 

 trict be not wholly kept down by such causes, — or again 

 let the destruction of eggs or seeds be so great that only a 

 hundredth or a thousandth part are developed, — yet of those 

 which do survive, the best adapted individuals, supposing 

 that there is any variability in a favorable direction, will tend 

 to propagate their kind in larger numbers than the less well 

 adapted. If the numbers be wholly kept down by the causes 

 just indicated, as will often have been the case, natural selec- 

 tion will be powerless in certain beneficial directions ; but this 

 is no valid objection to its efficiency at other times and in other 

 ways ; for we are far from having any reason to suppose 

 that many species ever undergo modification and improve- 

 ment at the same time in the same area." 



Some of the admissions made in this paragraph have an 

 important bearing on the theory of natural selection. Far 

 from supposing that fortuitous destruction would have no 

 influence on the course of natural selection, it can be shown 

 that it would have a most disastrous effect. In many cases 

 the destruction comes in the form of a catastrophe to the 

 individuals, so that small differences in structure, whether 

 advantageous or not, are utterly unavailing. Our experience 

 shows us that a destruction of this sort is going on around 

 us all the time, and accounts in large part for the way in 

 which the majority of animals and plants are destroyed. 

 Unless, for example, a seed happen to fall on a place suitable 

 for its growth, it will perish without respect to a slight advan- 

 tage it may have over other seeds of its kind. Of the thou- 

 sands of eggs laid by one starfish, chance alone will decide 

 whether one or another embryo is destroyed by larger animals, 

 or if they escape this danger, the majority of them may be 



