THE c:akkiers of heredity 



Many different lines of study have made it seem ])roljaljle that much, at least, of the heredity 

 of an a'nimal or plant is carried in the nucleus of the germ-cell and that in this nucleus 

 it is further located in little rods or threads which can be easily stained so as to become 

 visible, and which have the name of chromosomes. In the above illustration four 

 different views of the nucleus of the germ-cell of an earthworm are shown, with the 

 chromosomes in different stages: in section 19 each chromosome is doubled up like 

 a hairpin. Study of the fruit-fiy Drosophila has made it seem probable not only that 

 the hyj:)othctical factors of heredity are located in the chromosomes, but that each 

 factor has a perfectly definite location in its chromosome; and Dr. Alorgan and his 

 associates have worked out an ingenious means of measuring the distance from cither 

 end, at which the factors lie. Photomicrograjjh from P\)ot and StrobcU, Archiv. f. 

 Zellforschung V, pi. xii. (Fig. 8.j 



at that time uncxijlainaljlc. We have 

 now thrown out the Lamarckian view, 

 and if we have not been able really to 

 exi)lain how variations arise, we have at 

 least, Dr. Morjijan thinks, a truer view 

 of what happens. His discussion of 

 this subject is ])erhai)S the least satis- 

 factory part of the book, h)ut his posi- 



tion is clear enouj.^h, being based on the 

 developments of Mendelian heredity. 



NATURE OF VARIATIONS 



Darwin thought of the individual as a 

 unit, which was undergoing variations 

 in all ])arts. As a result of exi)erinicntal 

 breeding. Dr. Morgan says, we must now 



