INTRODUCTION 



13 



fers its hereditary tendencies from generation to generation, at first 

 unchanged, and always uninfluenced in any corresponding manner, 

 by that which happens during the life of the individual which bears 

 it. If these views be correct, all our ideas upon the transformation 

 of species require thorough modifica^tion, for the whole principle of 

 evolution by means of exercise (use and disuse) as professed by La- 

 marck, and accepted in some cases by Darwin, entirely collapses " 

 {I.e., p. 69). 



It is impossible, he continues, that acquired traits should be trans- 

 mitted, for it is inconceivable that definite changes in the body, or 

 "soma," should so affect the protoplasm of the germ-cells as to cause 

 corresponding changes to appear in the offspring. How, he asks, 

 can the increased dexterity and power in the hand of a trained piano- 

 player so affect the molecular structure of the germ-cells as to pro- 

 duce a corresponding development in the hand of the child t It is 

 a physiological impossibility. If we turn to the facts, we find, Weis- 

 mann affirms, that not one of the asserted cases of transmission of 

 acquired characters will stand the test of rigid scientific scrutiny. It 

 is a reversal of the true point of view to regard inheritance as taking 

 place from the body of the parent to that of the child. The child 

 inherits from the parent germ-cell, not from the parent-body, and the 

 germ-cell owes its characteristics not to the body which bears it, but 

 to its descent from a preexisting germ-cell of the same kind. Thus 

 the body is, as it were, an offshoot from the germ-cell (Fig. 5). As 



- _5' Line of succession. 



(*) Line of inheritance. 



G 



Fig. 5. — Diagram illustrating Weismann's theory of inheritance. 



G. The germ-ceil, which by division gives rise to the body or soma {S) and to new germ-cells 

 ( G\ which separate from the soma and repeat the process in each successive generation. 



far as inheritance is concerned, the body is merely the carrier of the 

 germ-cells, which are held in trust for coming generations. 



Weismann's subsequent theories, built on this foundation, have 

 given rise to the most eagerly contested controversies of the post- 

 Darwinian period, and, whether they are to stand or fall, have played 

 a most important part in the progress of science. For aside from thej 

 truth or error of his special theories, it has been Xy^isnianiLS great 

 service to place the keystone between the work of the e^lutionists 

 an d tha t of the cytologists, and thus to bring the cell-theory and the 



