76 REMARKS ON CERTAIN [XI. 



to a stand-still ? To my mind the descent of the immortal to the 

 condition of mortality, is less to be marvelled at than the fact 

 that monoplastids and germ-cells have remained immortal. 

 The slightest change in the properties of living matter might 

 involve such a descent, and certain essential peculiarities in 

 the composition of this substance must be most rigidly main- 

 tained, in order that the metabolic cycle may sweep on w^ith 

 perfect smoothness, and raise no obstacle against its own per- 

 sistence. Even if we know nothing further of these essential 

 peculiarities of structure, we may at least maintain that the 

 rigorous and unceasing operation of natural selection is neces- 

 sary to maintain them. Any deviation from this standard ends 

 in death. I believe that I have shown that organs which have 

 ceased to be useful become rudimentary, and ultimately dis- 

 appear owing to the principle of panmixia alone, — not be- 

 cause of the direct effect of disuse, but because natural selec- 

 tion no longer maintains them at their former level. What is 

 true of organs is also true of their functions ; for function is 

 but the expression of certain peculiarities of structure, whether 

 we can directly perceive the connection or not. If then the 

 immortality of unicellular beings rests on the fact that the struc- 

 tural arrangement of their substance is so accurately adjusted 

 that the metabolic cycle always comes back to the same point, — 

 why should, or rather, how could this property of the proto- 

 plasm, which is the cause of immortality, be retained when it 

 ceased to be necessary ? And clearly it is no longer of use in 

 the somatic cells of heteroplastids. From the moment that 

 natural selection relaxed its hold upon this property of the 

 protoplasm, the power of panmixia began to be felt, and ulti- 

 mately led to its disappearance. Prof Vines will probably ask 

 how this process can be conceived. I answer, quite simply. 

 Let us suppose that certain individuals appeared among the 

 monoplastids with such variation of the chemical or molecular 

 characters, that the continuous recurrence of their metabolic 

 cycle came to an end, so that natural death became a necessity. 

 These individuals could never give rise to a persistent variety. 

 But if individuals with a similar variation in their somatic cells 

 arose among the heteroplastids, no detriment would be felt by 

 the species : the body-cells would indeed die, but the undying 

 germ-cells would secure the continuance of the species. By 



