Nature of Heredity. ^zg 



see, in no other way. We may suppose that the original nucleus 

 divides into two halves of precisely equal dynamical value, each of 

 which, therefore, requires the same form of cell-hody to equilibrate 

 it, and the onlv wav in which this equilibration c-an be attained is 

 by the development (amongst other things) of a new flagellum by 

 one of the daughter cells. In other words, the nucleus exercises a 

 definite controlling force over the cell-body of such a character as 

 to evoke a definite response in the form of structural modification. 



We have here a simple case of heredity, and, as I believe, one 

 of inheritance of acquired characters. There can be little doubt 

 that the evolution of the flagella in Hcteromita was due in the first 

 instance to the direct action of the environment. We know how 

 readily in Amccba temporary pseudopodia are emitted when the proto- 

 plasm is appropriately stimulated, and the transition from pseudo- 

 podia to flagella is a perfectlx gradual one. At first temporary, 

 these organs gradually became by more and more frequent use per- 

 manent structures. Their development must have disturbed the pre- 

 •existing balance of forces between the cell-body and its nucleus, but 

 as it probably took place very gradually, the process extending over 

 many generations, this disturbance was not sufficient to produce 

 disruption, and the forces in the nucleus became slowly re-arranged 

 in equilibrium with the changing structure of the cell-body. Thus 

 in turn the nucleus acquired a new potentiality, a tendency to compel 

 .the cell-body to produce a flagellum in order to equilibrate its own 

 stored up forces. In other words, the development of a flagellum 

 by the cell-body acts as a stimulus upon the nucleus, and this 

 stimulus is stored up in the nucleus and given out again subsequently 

 •to the cell-body, inducing the latter to develop a flagellum when 

 necessary to restore the equilibrium between cell and nucleus. Thus 

 in time the production of the flagellum comes to partake of the 

 nature of an after-effect which may take place independently of the 

 environment. 



It may be urged that the development of the new anterior 

 flagellum by the young Heteromiia is due solely to that same action 

 ■of the environment which originally called forth the flagellum in 

 the ancestors of the race. The action of the environment, so long 

 as the conditions remained the same, would certainly tend to pro- 

 duce the same effect in each generation, but the conditions do not 

 remain exactly the same, for the new flagellum appears before its 

 possessor commences to lead an independent life. Moreover, it 

 always appears in such a definite fixed position and with such 

 rapidity and precision that we cannot believe that it is evoked 

 de novo by the action of the environment in the development of each 

 individual. In the higher animals, again, there are many characters 

 which appear regularly in the development of the individual, and 

 which no longer have any relation whatever to the nature of the 

 -environment ; such, for example, are the gill-slits in the neck of the 

 embr}o chick, and these can be due only to the action of internal 

 forces. 



