352 ACTION OF NATURAL SELECTION 



Evolution is brought about by the action of Natural 

 Selection on variations, it selecting some and rejecting 

 others, and so gradually altering the average char- 

 acters of the race; but are blastogenic or germinal 

 variations alone of value to such a selective agency, 

 and are somatic variations, or so-called acquired char- 

 acters, valueless in this respect ? As is well known, the 

 question of the heritableness of acquired characters has 

 been one of the most hotly debated of all biological 

 problems, and is one which even now separates most 

 biologists into two opposite and apparently irreconcil- 

 able camps. It behoves us, therefore, to see if we can- 

 not find some via media, which, though unable to ad- 

 mit of the heritableness of localised tissue changes 

 such as injuries and mutilations, is yet able to adopt 

 reasonable evidence, both experimental and theoretical, 

 in favour of a partial inheritance of certain general- 

 ised tissue changes. 



The chief experimental evidence in support of the 

 apparent heritableness of acquired characters lies in the 

 numerous and undoubted proofs of the cumulative 

 action of conditions of life. Of such proofs, one of the 

 most striking is that recorded by Darwin * with refer- 

 ence to the effects of a European climate on the Ameri- 

 can varieties of maize. Thus Metzger cultivated in 

 Germany a tall kind of maize, Zea altissima, brought 

 from the warmer parts of America, and, " During the 

 first year the plants were twelve feet high, and a few 

 seeds were perfected. . . In the second generation the 

 plants were from nine to ten feet in height, and 

 ripened their seed better. . . Some of the seeds had 

 * " Animals and Plants," i. p. 340. 



