v.] IN THE THEORY OF NATURAL SELECTION. 285 



organisms. In these latter the distinction between body-cell 

 and germ-cell does not exist. Such organisms are reproduced 

 by division, and if therefore any one of them becomes changed 

 in the course of its life by some external influence, and thus 

 receives an individual character, the method of reproduction 

 ensures that the acquired peculiarity will be transmitted to 

 its descendants. If, for instance, a Protozoon, by constantly 

 struggling against the mechanical influence of currents in 

 water, were to gain a somewhat denser and more resistent 

 protoplasm, or were to acquire the power of adhering more 

 strongly than the other individuals of its species, the peculiarity 

 in question would be directly continued on into its two de- 

 scendants, for the latter are at first nothing more than the two 

 halves of the former. It therefore follows that every modifica- 

 tion which appears in the course of its life, every individual 

 character, however it may have arisen, must necessarily be 

 directly transmitted to the two offspring of a unicellular 

 organism. 



The pianist, whom I have already used as an illustration, 

 may by practice develope the muscles of his fingers so as to 

 ensure the highest dexterity and power ; but such an effect 

 would be entirely transient, for it depends upon a modification 

 in local nutrition which would be unable to cause any change 

 in the molecular structure of the germ-cells, and could not 

 therefore produce any effect upon the offspring. And even if 

 we admit that some change might be caused in the germ-cells, 

 the chances would be infinity to nothing against the production 

 of the appropriate effect, viz. such a change as would lead to 

 the development in the child of the acquired characters of the 

 parent. 



In the lowest unicellular organisms, however, the case is en- 

 tirely different. Here parent and offspring are still, in a certain 

 sense, one and the same thing : the child is a part, and usually 

 half, of the parent. If therefore the individuals of a unicellular 

 species are acted upon by any of the various external influences, 

 it is inevitable that hereditary individual diflerences will arise 

 in them ; and as a matter of fact it is indisputable that changes 

 are thus produced in these organisms, and that the resulting 

 characters are transmitted. It has been directly observed that 

 individual differences do occur in unicellular organisms, — dif- 



