1G8 OUTLINES OF EVOLUTIONARY BIOLOGY 



to be made up of "biophors," which are themselves the lowest 

 vital units, but each of which is in turn made up of molecules in 

 the chemical sense of the term ; while, on the other hand, the 

 determinants are supposed to be grouped in " ids," each of which 

 is a complete ancestral germ plasm, theoretically sufficient in 

 itself to determine all the different characters of an entire 

 individual. The ids may in some instances correspond to the 

 chromosomes, but these appear generally to be composite bodies 

 (" idants ") made up each of a large number of ids (chromomeres). 

 The determinants, and of course the biophors also, are far below 

 the limits of visibility even with the aid of the most powerful 

 microscope ; the ids, however, frequently appear during the 

 process of mitosis and may give the spireme thread or the chro- 

 mosomes into which it divides a characteristic beaded appearance 

 (Fig. 32, B). Biophors, determinants, ids and idants must all be 

 looked upon as living entities, growing by the absorption of 

 nutriment and multiplying by division. 



In accordance with Weismann's theory the germ cells them- 

 selves may be regarded as so many unicellular organisms, which 

 multiply by fission and periodically, if they chance to meet with 

 mates, conjugate with one another. Their cytoplasm as well as 

 their chromatin is directly continuous from generation to genera- 

 tion just as it is in a dividing Amoaba, and theoretically there is 

 no reason why the constant succession of germ cells should ever 

 be interrupted by death. The soma, or body, however, stands in 

 a very different position. It may be regarded as a kind of 

 appendage thrown off from the chain of germ cells after each 

 conjugation, and resulting from the fact that most of the cells 

 arising from the segmentation of the zygote not only remain 

 together in intimate association with one another but become 

 specialized in various directions and co-operate with one another 

 to form a complex multicellular individual. Having exhausted 

 its powers of growth and renewal this individual body sooner or 

 later dies ; but the germ cells periodically renew their powers of 

 cell-division by conjugation and, under favourable conditions, go 

 on for ever. 



According to Weismann, inherited characters are transmitted 

 not from soma to soma but from germ cell to germ cell, by virtue 

 of the continuity of the germ plasm. The soma has little if any 

 influence upon the germ cells which it contains beyond that which 

 is involved in supplying them with protection and nourishment. 



