388 ^'iVlC BIOLCXiY 



Evolution, mutation, and Mendel's law. In his scheme of evohitioii 

 Darwin emphasized the influence of slight variations continued through 

 long periods of time. He realized at the outset that in heredity, in the 

 i)Ower to pass on variations, lay the heart of his problem, but he went 

 far astray in his own theory of heredity, pangenesis,^ and so failed to 

 attain the goal he might have won. Xo one realized this more keenly 

 than Darwin himself. 



De Vries found that from the same seed capsnle of Lamarck's eve- 

 nino- primrose he could rear as many as nine distinct kinds of plants, so 

 different that, liad they occurred consistently in nature, they might 

 have been named as separate species. On the basis of these and similar 

 experiments he advanced his recent theory of mutation. This theory 

 supposed that evolution goes forward by leaps and sudden changes. It 

 now tnrns out that this evening primrose, (Enothera JaniarckUma, is 

 a Mendelian cross, a hybrid; and this suggests that all mutations may 

 be merely cases of segregation and recombination of nnit characters in 

 the germs of plants and animals, that is, outworkings of Mendel's law. 



Weismann made a solid contribution when he distinguished sharply 

 between germ plasm and body plasm, or somatoplasm. He called atten- 

 tion to the fact that the germs are all formed in the embryo lonf/ 

 before the body; the egg-germs, and many more than a hen can ever 

 hope to lay, are all set aside at almost the very beginning of incubation. 



1 Pangenesis {pan, "all,'' or '''the whole,^' and genesis, " origin '" — that 

 is, "from the whole body") is the theory that the germ cells are built up 

 by the streaming together, from all the organs of the body, of minute parti- 

 cles (gemmules, or pangens) — an infolding or involution of the body into 

 the germ. Then when a germ unfolds or develops, each pangen reproduces 

 the part of the body from which it came. This theory implies an active 

 influence of the body upon the germ plasm, and if parts of the body or brain 

 should be specially developed by exercise or training, or if parts or organs 

 should be removed or lost by disease or accident, we should expect to find 

 such additions or subtractions reproduced when the germs from such bodies 

 developed. This we never find. There is no evidence that any acquired 

 character is ever inherited. Lambs' tails have iTeen bobbed for thousands 

 of years, and lambs are born with tails as long as they ever were. Galton 

 disproved pangenesis experimentally by exchanging the blood of animals. 

 Since the blood is the only means by which the pangens could possibly 

 circulate from the body to the reproductive cells, if we exchange blood 

 between white and black animals, we ought to get some of the pangens 

 mixed. Galton's experiments disproved the theory absolutely, as does every 

 case of budding and grafting. 



