170 OUTLINES OF E VOLUTION AEY BIOLOGY 



elongated in form. If, as Weismann maintains, and as can 

 actually be demonstrated in many cases, each elongated chromo- 

 some is made up of a row of chromomeres or ids, which may be 

 supposed to differ to some extent from one another as to the 

 determinants which they contain, it is obvious that longitudinal 

 splitting of the chromosome is the only way in which a qualita- 

 tive as opposed to a mere quantitative division of the hereditary 



YIG. 76. Diagram to illustrate the contrast between Darwin's Theory of 

 Pangenesis and Weismann's Theory of the Continuity of the Germ 

 Plasm. 



The figures represent an imaginary organism with four processes given off from the soma 

 or body, which is supposed to contain only a single germ cell (dotted). Three genera- 

 tions are represented, and for the sake of simplicity the complication introduced 

 by the periodical conjugation of male and female germ cells is omitted. Figs. A, 

 B, C show how an acquired character the elongation of one of the processes as a 

 result of its use for some special purpose may be supposed to affect the germ cells 

 through the migration of gemmules (indicated by the small arrows), and thus be 

 transmitted by heredity in accordance with the theory of Pangenesis. Figs. A', B', 

 C' show how such an acquired character is, in accordance with Weismann's theory, 

 unable to make any impression upon the germ cells and is therefore not transmitted 

 by heredity. In the first case the germ cells of each generation are supposed to arise 

 from the soma; in the second case they are supposed to arise directly from the 

 preceding generation of germ cells, which also gives rise to the soma in which they 

 are enclosed, as indicated by the large arrows. 



substance can be brought about. This mode of division, then, 

 otherwise difficult to explain, is fully intelligible in accordance 

 with Weismann's theory. 



It is not necessary to suppose, however, that such division 

 always results in identical daughter chromosomes. We may 

 assume either that all the individual ids divide into similar 

 halves, containing similar determinants (as represented very 

 diagrammatically in Fig. 77, A), in which case the two daughter 

 chromosomes will be exactly alike, or that some or all of the ids 



