INTRODUCTION I I 



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 produce 

 a corresponding development in the hand of the child ? It is a physi- 

 ological impossibility. If we turn to the facts, we find, Weismann 

 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 pre-existing germ-cell of the same kind. Thus 

 the body is, as it were, an offshoot from the germ-cell (Fig. 4). As 



" .S Line of succession. 



Line of inheritance. 



Fig. 4. Diagram illustrating Weismann's theory of inheritance. 



G. The germ-cell, 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 the 

 truth or error of his special theories, it has been Weismann's great 

 service to place the keystone between the work of the evolutionists 

 and that of the cytologists, and thus to bring the cell-theory and the 

 evolution-theory into organic connection. It is from this point of 

 view that the present volume has been written. It has been my 

 endeavour to treat the cell primarily as the organ of inheritance and 

 development; but, obviously, this aspect of the cell can only be 

 apprehended through a study of the general phenomena of cell-life. 

 The order of treatment, which is a convenient rather than a strictly 

 logical one, is as follows : 



The opening chapter is devoted to a general sketch of cell-struct- 



