TRANSMISSION OF MODIFICATIONS 417 



The probability is that they are entirely local, sometimes arising 

 from external injuries, as in galls, and thus outside the present 

 field of inquiry, but more often due to internal disturbances in 

 the cells themselves at the particular point where the abnormal 

 functioning appears. 



Modifications due to external causes sometimes transmitted, 

 oftener not. The effects of such all-pervading influences as 

 nourishment, temperature, chemical action, gravity, etc., are 

 not felt simply by the external parts, but on the contrary they 

 may extend to the innermost parts of the organism, affecting 

 the constitution and the activities of the most highly specialized 

 matter, extending, for all we know, to the very germ, in which 

 case they would certainly be transmitted, whereas there is no 

 ground for belief in the transmission of influences that do not 

 affect the germ. 



Summary. When we speak of the transmission of a modifi- 

 cation we mean rather the transmission of a character as modi- 

 fied. Strictly speaking, a character will be transmitted in a 

 modified form if tJie modification affects the germ ; otherwise it 

 will be transmitted in an unmodified form, for the germ is the 

 only hereditary substance, and nothing is transmitted except 

 through the germ. 



* A modification may arise from causes either internal or exter- 

 nal to the germ. If internal it of necessity affects the germ and 

 is transmitted. This is the ordinary cause of hereditary varia- 

 bility. If, on the other hand, the modification arose from external 

 causes, the germ may or may not be affected, and the modifica- 

 tion may or may not be transmitted. 



There is much reason to believe that many modifications of 

 functional activity are of such a fundamental nature as to influ- 

 ence the germ as well as the soma, and such modifications 

 would be transmitted and inherited by the offspring. 



In general, the great effect of the environment is to influence 

 development, not to induce new characters. The environment 

 does not decide what characters shall compose the individual, 

 that is a matter of straight inheritance, but it does decide 

 to a very large extent what degree of development the various 

 racial characters shall attain in particular individuals. The 



