48 THE GERM-PLASM 



by primordial generation : all subsequent and more complex 

 kinds of hiophors can only Jiave arisen on tJie principle of 

 adaptation to new conditions of life; they must have been 

 developed gradually by the long -continued co-operation of 

 heredity and selection. All these biophors of a higher order, 

 which are adapted to the special conditions of existence and 

 which in endless varieties form organisms as we see them around 

 us, possess 'historical' qualities; they can. therefore, only arise 

 from others like themselves, and cannot be formed spontaneously- 

 This fact is confirmed by experience. Not only does a cell 

 always arise from a cell, and a nucleus from a nucleus, as de 

 Vries. and more recently Wiesner, have shown, but all the other 

 constituents which occur in the cell-body and determine its 

 structure never arise, so far as we know, by ^ generatio equivocal 

 or, as de Vries expresses it, ' neogenetically.' They are always 

 produced by the division of similar structures already present. 

 This is apparently true of the green chromatophores and the 

 H-acuoles"' of plant-cells, as well as of the ^sphere of attraction,' 

 or centrosome, which controls the division of the nucleus : the 

 same must also hold good for those invisible vital units, the 

 various kinds of biophors, which have arisen during the course 

 of the earth's history by gradual adaptation to continually new 

 conditions of life. 



If then, each vital unit in all organisms, from the lowest to the 

 highest grade, can only arise by division from another like itself, 

 an answer is given to the question with which we started ; and 

 we see that the structures of a cell-body, which constitute the 

 specific character of the cell, cannot be produced by the emitted 

 influence of the nuclear substance, nor by its enzymatic action, 

 but can only arise owing to the migration of material particles 

 of the nucleus into the cell-body. Hence the nuclear niatter 

 must be in a sense a storehouse for the various kinds of biophors 

 which enter into the cell-body and are destined to transfor)n it. 

 Thus the development of the 'undifferentiated' embryonic cell 

 into a nerve-, gland-, or muscle-cell, as the case may be, is 

 determined in each case by the presence of the corresponding 

 biophors in the respective nuclei, and in due time these 

 biophors will pass out of the nuclei into the cell-bodies, and 

 transform them. 



To me this reasoning is so convincing that any difficulties we 

 meet with in the process of determining the nature of the cell 



-J 



