INHERITANCE OF PARENTAL MODIFICATIONS 79 



example, animals living in cold countries have thicker fur 

 than tropical species. This might readily be brought about 

 by natural selection, but we have little to indicate that the 

 direct effect of cold upon each individual is such as to cause 

 increased thickness of hair. 



One more question naturally presents itself. If changes 

 in the offspring are not produced by changes in the body 

 (sonia] of the parent, how do variations come to appear in 

 the offspring ? Variations arise in the germ cells and are 

 transmitted from them to their offspring. Changes in the 

 internal constitution of the germ cells will cause changes to 

 appear in the young which arise from these germ cells. The 

 character of every animal or plant is dependent upon the 

 character of the s^erm cell from which it comes. 1 A new- 



O 



laid egg of a chicken almost exactly resembles a new-laid egg 

 of a duck. The most careful study of the two would not 

 show any trace of the differences which are to appear as the 

 eggs develop ; yet it must be that the two eggs differ in their 

 constitution and that to this difference in structure is due the 

 difference between the birds which will hatch from the two 

 eggs. The character of the adult is predetermined by the 

 character of the egg. Of course, then, anything which 

 causes changes in the character of the egg may cause 

 correlated changes in the adult which is developed from 

 the egg. 



But what can cause such changes in the egg or spermato- 

 zoon ? It lies inside the body of the animal or plant which 



1 This is equally true whether we believe with Weismann that every organ of the 

 future adult is represented by a corresponding differentiated though minute particle 

 in the germ, or with Hertwig that the germ cell is more nearly homogeneous, differ- 

 entiation appearing as growth proceeds. 



