194 AMPHIMIXIS OR ESSENTIA I MEANING OF [XII. 



a certain external form and limit of size, which it will regain 

 after being mutilated. Growth and division are themselves the 

 outcome of such tendencies implanted in the molecular structure: 

 this, for example, is the case in Bacteria. The whole question 

 comes to an end when we reach those lowest of all beings, 

 which are entirely formless and have no fixed size, — beings 

 which we must regard, little as we know about them, as crossing 

 the very threshold of organic life'. 



It is interesting to observe that, from this point of view, the 

 nucleus presents itself in a new light. By the agency of con- 

 jugation and fertilization it becomes an organ for maintaining the 

 constant renewal and transformation of hereditary individual vari- 

 ability. Besides this, it plays the part of protecting the species 

 from the too powerful effect of transforming external influences 

 upon the body, inasmuch as it tends to prevent these from 

 becoming hereditary, not indeed actively, but simply because 

 every external influence does not cause a corresponding altera- 

 tion in the nuclear substance, and thus the latter containing the 

 older predispositions tends to restore, after each fission, the older 

 condition of the cell-body. It simultaneously acts as a conserv- 

 ing and as a progressive principle, exactly as the sexual cells of 

 higher beings are, according to my views, supposed to behave. 

 The reproductive cells exert a conserving force, inasmuch as 

 they retain, with incredible tenacity, the hereditary tendencies 

 contained within them, and, above all, because they are unaffected 

 by those changes in the soma which are brought about by external 

 influences : but they also act progressively by means of amphi- 

 mixis and the consequent periodical minghng of the hereditary 

 predispositions of two germ-cells, one from each parent, which 

 as we have seen, takes place by the removal of half of these 

 predispositions and by the arrangement of those which remain 

 in fresh combinations. 



If I am correct in my view of the meaning of conjugation as a 

 method of amphimixis, we must believe that all unicellular 

 organisms possess it, and that it will be found in numerous low 

 organisms, in which it has not yet been observed. But it is by 

 no means safe to make the a priori assumption that conjugation 



^ Nageli, ' Mechanisch-physiologische Theorie der Abstaramungslehre.' 

 Munich, 1884. 



