278 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



higher organisms the Metazoa and Mctaphyta; hut it is to be 

 sought for in the lowest the unicellular 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 descendants, for the 

 latter are at first nothing more than the two halves of the former. 

 It therefore follows that every modification which appears in the 

 course of its life, every individual character, however it may have 

 arisen, must necessarily be directly transmitted to the two off- 

 spring 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 differences 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, differences in size, 



