HEREDITY 483 



mission of exogenous somatic modifications. But several 

 biologists for whom we have the greatest respect think other- 

 wise, and, without any indecision on our own part, we would 

 refer to the works cited in the bibliography under the names 

 of Hartog, MacBride, and Semon. 



§ 3. Modifications and Heredity. 



In the absence of any convincing evidence that exogenous 

 modifications acquired by parents can be transmitted to their 

 ofi'spring, either as such or in any representative degree, we 

 have to face the question whether individual modifications 

 have any evolutionary interest at all. It may be answered, 

 first, that deeply-saturating modifications may influence the 

 blood and other fluids of the body, or may alter the rhythm 

 of metabolism so that the production of internal secretions 

 is affected, and that these internal changes in the somatic 

 environment may act as liberating stimuli on the germ- 

 plasm and provoke variations. Prolonged exercise, e.g., in 

 dancing, may lead to an exaggerated production of muscle- 

 forming substance; the myogenic metabolism may be en- 

 hanced; this may be spread through the body, e.g., from 

 limbs to heart ; it is conceivable that it might affect the germ- 

 plasm specifically. 



The second answer is that suggested by Profs. IMark 

 Baldwin, Lloyd Morgan, and H. F. Osbom, that an adaptive 

 modification may serve as a protective screen for the indi- 

 vidual until, perhaps, a germinal variation in the same direc- 

 tion has time to arise and establish itself. What is not 

 organically entailed may be acquired afresh in each suc- 

 cessive generation. In an area where a dark skin was of 

 survival value, acquired tanning might save many in- 

 dividual lives until, perhaps, a germinal variation in the 



