8 THE PRINCIPLES OF HEREDITY 



13. A variation, then, is a new inborn character. It is a 

 new character, but it is not an acquirement. A modification 

 is an acquirement, but it is not necessarily a new character, 

 for many modifications have been acquired by every genera- 

 tion for thousands of years. Moreover, while a variation 

 implies a difference from the parent, a modification may 

 imply a likeness, since both the parent and the child may 

 acquire the same modification. Modifications, as implied in 

 the definition of them, are alterations of inborn characters 

 caused by the play of forces from the environment on the 

 soma of the individual that makes them. 1 The cause of 

 variations is more obscure. Several hypotheses to account 

 for them have been formulated, all of which are unsatis- 

 factory as inadequate to explain all the facts, or as opposed 

 to ascertained facts, or as founded on suppositions which 

 cannot be verified. The whole problem of heredity centres 

 round this question of the causation of variations. It lies at 

 the starting-point of every conceivable doctrine of evolution ; 

 for every inborn character, as such, must have appeared in 

 the first instance as a variation, and the evolution of every 

 race has depended on the variations of its individual 

 members. But it is of more than merely scientific interest. 

 Accordingly as we take this view or that we shall form a 

 conception of life and of certain great practical problems, 

 which press for solution in the near future, entirely and 

 fundamentally opposed to that which we would otherwise 

 hold. 



14. Congenital likenesses and differences between parents 

 and offspring depend on antecedent likenesses and differences 

 between the portions of germ-plasm contained in the germ- 

 cells from which they arise respectively. Every theory of 

 heredity consists in an attempt to explain why these portions 

 of germ-plasm are somewhat alike, but not quite alike. The 

 various explanations of the likenesses may be divided into two 

 categories. One set of theories supposes that the germ-plasm 

 contained in the germ-cells of the parent is compounded of 

 units derived from all his parts. Thus Darwin in his theory 

 of " Pangenesis " supposed that every cell of the body throws 



1 This statement perhaps requires qualification. As we shall see later 

 ( 275), there is some reason to believe that modifications may be due 

 occasionally to the play of environmental forces, not on the soma, but 

 on the germ-cell itself. Presumably the germ-cell, but not the germ- 

 plasm contained within it, is then modified. Changes appear in the indi- 

 vidual who springs from the cell, but, since the hereditary tendencies 

 of the race are not altered, they are not transmitted to offspring. 



