XII.] CONJUGATION AND SEXUAL REPRODUCTION. 1 93 



animals. Clearly it has arisen from the necessity of providing 

 the process of natural selection with a continually changing 

 material, by the combinations of individual characters. 



Amphimixis in all Unicellular Organisms. 



We may extend this conception and enquire w^hether it may 

 not, in reality, apply to all unicellular organisms, that is all 

 v^hich possess a nucleus and cell-body. The conclusion can 

 scarcely be avoided if it be admitted that the nucleus invariably 

 bears the same essential significance, and this can hardly be 

 doubted. If, as a matter of fact, the lowest, apparently struc- 

 tureless unicellular organisms contain a nuclear substance which 

 dominates and controls the entire animal, it follows that all 

 lasting and therefore hereditary variations of both cell-body 

 and nucleus must proceed from the latter, while those direct 

 changes of the cell-body which are produced by external in- 

 fluences, are as incapable of hereditary transmission as the 

 mutilation of the body of an Infusorian. Thus changes in the 

 molecular constitution of the cell-body, such as we might 

 imagine to be the result of the exercise of particular functions 

 (for example, the more powerful movements of an amoeba) would 

 probably be transmitted to the immediate offspring, but would 

 disappear with the cessation of those causes which rendered 

 necessary the increased exercise of the function concerned. 



My earlier views on unicellular organisms as the source of 

 individual differences, in the sense that each change called forth 

 in them by external influences, or by use and disuse, was sup- 

 posed to be hereditary, must therefore be dismissed to some 

 stage less distant from the origin of life. I now believe that 

 such reactions under external influences can only obtain in 

 the lowest organisms which are without any distinction between 

 nucleus and cell-body. All variations which have arisen in 

 them, by the operation of any causes whatever, must be inherited, 

 and their hereditary individual variability is due to the direct 

 influence of the external world. Loss of substance must not 

 however be included among such individual variations : repair 

 would take place by regeneration in these simplest forms of life 

 just as in higher Protozoa. At least, I think this is not con- 

 tradicted by the fact that the molecular structure of such a 

 Moneron, although without the guidance of a nucleus, retains 



VOL. II. 



