.)/ 



(\ THE EVOLUTION THEORY. 



l.ioplior is ultiimiti'ly only a group of molecules, the determinants 

 a group of hiopliors, the id a group of determinants, and all the three 

 inferred sta^i-es of vital organization only become real units through 

 the forces oi)eratiii<j: within them and combining them into a whole. 

 What compels the chromatin granules of the resting nucleus to 

 apinoac'h each other at the time of cell-division, to unite into a long, 

 band-like thread, and what is it that subsequently causes this thread 

 to break up again into a definite number of pieces 1 (J])viously only 

 internal forces of which we know nothing further than that they are 

 operative. 



We shall see later that this assumption of vital affinities must be 

 made not only in regard to the cells, but also in regard to entire 

 organisms whose parts are united by an internal bond, and wdiose 

 co-ordination is regulated by forces of which w'e have as yet no 

 secure knowledge. In the meantime we may designate these forces 

 by the name of ' vital affinities.' 



It must be admitted, however, that some objections of a funda- 

 mental nature have been urged against the assumption of a differential 

 nuclear division of the hereditary substance. O. Hertwig holds that 

 the assumption of differential division is essentially untenable, because 

 it is conti-adictory to ' one of the first principles of reproduction,' for 

 ' a phj^siologically fundamental character of every living Ijeing is the 

 power of maintaining its species.' 



This certainly seems so, but a closer examination shows that this 

 'principle,' although correct enough when taken in a very general 

 sense, does not really cover the facts, and is therefore incapable of 

 supporting the inferences drawn from it. If the proposition expressed 

 the whole truth there could have been no evolution from the primi- 

 tive organisms to higher ones, every living being must have simply 

 reproduced exact copies of itself. Whether the transformations of 

 species have been sudden or gradual, whether they have been brought 

 about by large steps or by very small ones, they could only have 

 come about by breaking through this so-called ' principle ' of like 

 begetting like. In fact, we may with more justice maintain the 

 exact converse of the principle, and say that ' no living being is able 

 to produce an exact copy of itself,' and this is true not only of sexual, 

 but of asexual reproduction. 



In ontogeny we see exactly the same thing. There are no two 

 daughter-cells of a mother-cell which are exactly alike, and the 

 differences between them, if they increase in the same direction, may 

 lead in later descendants to entire differences of structure. Indeed 

 the whole process of development depends on such an augmentation 



