46 THE PRINCIPLES OF HEREDITY 



The individuals that sprang from the more susceptible germ- 

 plasm would be less in harmony with the environment than 

 the others, and would be weeded out in greater numbers. 

 The conditions of life, therefore, are such that Natural Selec- 

 tion invariably tends to establish in the germ-plasm a high 

 degree of insusceptibility to the direct action of the environ- 

 ment. It is not necessary to believe, however, that the 

 insusceptibility is absolute. Evolution is never perfect. It 

 is necessary to believe only that a very high degree of 

 insusceptibility has been evolved in all races, a degree of 

 insusceptibility so high that only under very exceptional 

 circumstances, or in very exceptional germ-cells, does the 

 environment cause any change, a degree of insusceptibility so 

 high that the exceptions may be treated as negligible. 



79. To sum up : Influences from the environment acting 

 on the germ-plasm are causes of variations, or they are not. 

 If they are causes of variations, then every influence pro- 

 duces a definite type of variation, or it does not. If it pro- 

 duces a definite type of variation then Natural Selection is 

 rendered nugatory ; for, since the variations of the individuals 

 exposed to the influences would be in one direction, there 

 would be no scope for selection as regards that character. 

 The species would drift helplessly to destruction. There 

 would remain, besides, the difficulty, insurmountable under 

 this hypothesis, that the children of the same parents, not 

 only vary amongst themselves, but may vary in all directions 

 from the parents. Moreover, since definite variations caused 

 by influences from the environment can rarely be adaptive, it 

 is evident that Natural Selection must tend to eliminate from 

 the species that type of germ-plasm which is susceptible to 

 such influences, and so bring about a high degree of in- 

 susceptibility. The fact that some species, which are closely 

 adapted to an environment that has altered little, have ex- 

 isted almost unchanged for enormous epochs of time is proof 

 that the germ-plasm actually does possess a high degree of 

 insusceptibility. These species are still acted on by potent 

 influences, but they are not changed by them. The work of 

 Natural Selection is not rendered nugatory, and therefore they 

 are kept stable. On the other hand, if we suppose that given 

 influences cause variations, but not variations of a definite 

 type, then we must admit that the germ-cells must differ 

 amongst themselves, apart from any changes caused by the 

 action of the environment ; and, if we admit this, we admit 

 the existence of a sufficient cause for variations. If we seek 

 a further cause we violate the scientific principle of par- 



