THE ARGUMENT FROM DISEASE 167 



influences ; but under the normal conditions of existence such 

 a type is among the most unfit, and, therefore, Natural 

 Selection must tend constantly to purge the race of it. 



283. Many readers may have found the foregoing discussion 

 of the causes of variations very tedious. But the subject is 

 of surpassing importance. Except we have clear and correct 

 ideas concerning it we must go astray in our whole conception 

 of life. Only by means of such a discussion was it possible 

 to justify the position we have taken the negative position 

 that variations are not normally caused by the direct influence 

 of the environment. A negative position even when founded 

 on truth is always difficult and sometimes impossible to 

 establish. Had we maintained that external influences do 

 cause variations, we could easily have proved our case if 

 correct by instancing a few incontrovertible examples ; 

 and this is what those who disagree with us should do. All 

 nature is a field in which they may delve. Adopting the 

 opposite conclusion, a like course is not open to us. Had we 

 proved that this or that agency did not cause a change in the 

 germ-plasm in this or that instance, we should have been 

 told we were reasoning from the particular to the general. 

 What did not occur in one instance might occur in other 

 instances. Our only course, therefore, was to demonstrate 

 that under the conditions in which life exists it is impossible 

 that variations can be due, except only very rarely, to the 

 direct action of environment. The fact that species undergo 

 progressive evolution, not degeneration, when exposed to 

 influences which poison or otherwise enfeeble the soma and 

 the germ-cells is decisive. 



284. Of this at least we may be sure, that, if external 

 influences acting on the germ-plasm do alter offspring, then, 

 in the immense majority of instances, the change is not a 

 true variation, a true alteration of hereditary tendencies, but 

 merely an acquirement which affects the individual, but not 

 his offspring and descendants. 



