CAUSES OF VARIATION 101 



know only in a far-off sort of way, one individuality is very un- 

 likely to reproduce an absolute facsimile of itself. It is of the very 

 essence of a living thing to change, and an individuality cannot be 

 halved. From this point of view, variation is a primarily normal 

 occurrence, and breeding true has secondarily come about as the 

 result of restriction. In short, variability is a primeval character 

 of organisms. We cannot explain variability ; it is a datum in 

 the world of life. We may, however, try to show in certain cases 

 how it operates and what conditions help or hinder it. 



The unending problem of life is to establish some sort of modus 

 vivendi between an extremely complex and changeful animate 

 system and the extremely complex and changeful environment in 

 which it lives and moves and has its being. In all viable organ- 

 isms this equilibration has been established, and it is plain that 

 those organisms which could secure an entailment of this equili- 

 bration would be the organisms to survive. The producers of 

 survivable descendants survive in them — an obvious economy of 

 successful experiment, if such a point of view can be entertained. 



We have seen that during the early stages of development there 

 is often a visible segregation of a lineage of germ-cells which do not 

 share in body-making, but continue like the fertilised ovum. 

 This distinction between somatic cells which undergo differentia- 

 tion and germ-cells which retain the heritable qualities intact is 

 obviously an advantageous method of entailing on successive 

 generations that valuable asset which we have called organic 

 equilibration. It also economises and facilitates the process of 

 reproduction. 



But in spite of this almost universal device, the general tend- 

 ency of which is to secure persistence, continuity, and complete 

 hereditary resemblance, there is abundant opportunity left for 

 the assertion of that variability which we believe to be a primary 

 quality of vital units. Thus an inquiry into the causes of varia- 

 tion seems to us to be in the main an inquiry into the oppor- 

 tunities for the reassertion of a pristine tendency which the 



