THE INSUSCEPTIBILITY OF THE GERM-PLASM 93 



ment, the human being, they have been stable. They have not 

 drifted. 



149. There is nothing especially magical or mysterious about 

 this germinal power of resisting enforced change. There is 

 abundant evidence that it exists. If it did not exist no species 

 could persist. But, when we say that it exists, we do not imply 

 that the germ-plasm is incapable of being injured, incapable of 

 enfeeblement, or death, or alteration of its hereditary tendencies. 

 We imply merely that such enforced alteration of the germ-plasm 

 of a species that has been fitted by evolution to its environment is 

 usually an injury, and that the evidence is that when the injury is 

 severe enough to cause alteration of the hereditary tendencies, it is 

 usually severe enough to cause the death of the germ-plasm also. 

 In other words, we imply that the hereditary tendencies are 

 implanted in the germ-plasm as firmly, or almost as firmly as 

 life itself. Indeed collectively they are the life ; because of them 

 the individual (and the germ-plasm) absorbs nutriment, excretes 

 waste-material, grows, multiplies, and so forth. 



150. The power to resist enforced change is essential to 

 existence. Presumably, therefore, it is, like other essential 

 characters, a product of evolution. It is, at least, probable that 

 just as individual unicellular organisms vary as regards other 

 particulars, so they vary as regards the power of their germ-plasms 

 to resist enforced change. If this power be weak the germ-plasm 

 tends to be destroyed or altered by the influences to which it is 

 inevitably exposed. The descendants of the altered individuals, if 

 there be any, inheriting the weak resisting power, tend to be still 

 more changed and ultimately to be eliminated. But, if the 

 resisting power be great, and if individuals vary as regards it, 

 while at the same time spontaneous variations occur in all 

 directions, the race tends to survive ; for in that case there is no 

 inevitable drift ; natural selection has scope, and the resisting 

 power tends to grow stronger with selection. The conclusion we 

 reach, then, is that Natural Selection has established and maintains 

 the high insusceptibility of the germ-plasm to the direct action of 

 the environment. 1 



151. Turn, now, to multicellular animals and plants. Not only 



1 Interesting evidence of the stability of the germ-plasm of unicellular organ- 

 isms is furnished by Dr Pearl in his study of Paramcecium caudatum. These 

 organisms multiply for many generations without conjugation. But at intervals 

 conjugant individuals appear. " Dr Pearl grew Paramcecia under much variety 

 of environment, and found that the non-con jugant type was highly correlated 

 with the environment and the conjugant type singularly little affected by the 



