PRESENT PKOBLEMS IN EVOLUTION AND HEREDITY. 339 



llu' Lit'ini-cells ;ii'e cnlh'd blastouviu'tic, while jiccjuiicd characters aiis- 

 in.i? in the body cells are soinatogeuetic. 



To clearly understand this vie\y. let us follow the history of the fer- 

 tilized ovum in the Ibrniation of the embryo. It first divides into soma- 

 to-plasm and germ-pl»sm (see Fig. 1, Diagram iii), the former supplies 

 all the tissues of the body — m, s, w, dj r, nervous, muscular, vascular, 

 digestive, etc. — with their quota of hereditary structure; the residual 

 germ-plasm is kept distinct throughout the early process of eml)ryonic 

 cell division until it enters into the formation of the nuclei of the re- 

 productive cells, the ova or spermatozoa. Here it is isolated from 

 '•iianges of fuui'tiou in tlu^ somatoplasm, and in common with all other 

 protoplasm is capable of unlimited growth by cell division without loss 

 or deteriorati(m of its past store of hereditary i^roperties; these prop- 

 erties are lodged in the nucleus of each ovum and s])ermatozoan, and 

 tliese two cells, although widely different in external accessory struc- 

 ture (because they have to i)lay an active and passive part in the act 

 of conjugation), are exactly the same in their essential molecular 

 structure, and the ancestral characters they convey differ only because 

 they come along two different lines of descent. When these cells unite 

 they carry the germ-])lasm into the body of another individual. Thus 

 the somatoplasm of each individual dies, while the germ-plasm is im- 

 mortal; it simply shifts its abode from one generation to another; it 

 constitutes the chain from which the individuals are mere offshoots. 

 Thus the germ-plasm of man is continuous with that of all ancestors 

 in his line of descent, and we have an explanation of the early stages 

 observed in development in which the human embryo passes through 

 a succession of metamorphoses resembling the adult forms of lower 



types. 



In order to emphasize, as it were, the passage of the germ-plasm 

 from one generation to another without deterioration in its marvellous 

 hereditary ])owers, Weismann added the idea of its isolation. Not only 

 does he repudiate the pangenesis notion of increment of germ-plasm 

 by addition of gemmules, but he bidieves that it is unaffected by any 

 of the noriiuil changes in the somatic or body cells. As this contiiiuify 

 and isolation would r(;nder impossible the transmission of characters 

 accpiired by the somato-plasm, Weismann began to examine the evi- 

 <leiice for such transmission, and coming to the conclusion that it was 

 insuflicient, in his notable essay on "■Heredity," in 1SS3, he boldly a1 

 tecked the whole Lannirckian theory and has continued to do so in all 

 his snbse([uent essays. 



Being Ibrced to explain evolution witiuiut tiiis factor, he clainied 

 that variation in the germ plasm \\:is constanly arising by the union 

 of plasmata from different lines of descent in fertilization, and that 

 tliese variations are constantly being a(;ted upon by luitural selection 

 to produce new types. He thus revived Darwin's <'arlier views of 

 (•volution, and this in jiart e\[)lains his strong support by English nat- 

 uralists. 



