PRESENT PROBLEMS IN EVOLUTION AND HEREDITY, 335 



theories of heredity* •!• A. 'rii<)iii]»8«tii distiiijiuishes three <ieiieral 

 ])r<)l>h'iiis, which areofteii eoiitiised : 1st. \Vliat characters distinguish 

 the iierni cells from other cells of the body.' I'd. How do the };eriii 

 cells derive these distiii,uuishiii,ii' characters? '.k\. How shall we inter- 

 pret '"particulate" inheritance, or the re-appearance of single peculiari- 

 ties in the ortspringf 



The various theories may be grouped under two heads. •• I'angenesis 

 of Gerni cells" and ••Continuity of (ierni cells" according to the 

 dominating idea in ('ach. 



1. Pangenesis. — The idea ])ervading ]>angenesis was tirst expressed 

 !)>• Democritus that the ••seed" of animals was derived by contributions 

 of material jtarticles from all ]>arts of tlie bodies of both sexes, and that 

 like parts i)i-oduced like. Two thousand years latei-, Button revived 

 this conception of heredity in his ''Molecules organiques." In 1804 

 Herbert Spencer suggested the existence of "physiological niuts," 

 derived from the body cells of the parent, forming the germ cells and 

 then developing into the body cells of the offsi)ring. 



It is interesting to note the course of Darwin's thought upon this 

 matter in his published works and in his ''Life and Letters." He was 

 at first strongly opposed to the views upon evolution advance<l by 

 Butfon, by lOrasmus ])arwiu, his graiulfather, exi)anded by Lanuirck, 

 and now known as Tianuirckian. Hut gradually becoming convin<;ed 

 that his own theory of natural selection could not account for all the 

 facts of evolution, he unconsciously l»ecanie a strong advocate of 

 Lannirck's theory, and contributed to it a feature which Lanuuck had 

 entirely omitted, namely, a theory of heredity ex]>ressly designed to 

 explain the transmission of ac(iuired characters. Darwin's "provi- 

 sional hypothesis of i)angenesis " f ])ostulated a material connection 

 between tlie body cells and germ cells by the circulation of minutebuds 

 from each cell: each body cell throws off a "geiinnule" containing its 

 characteristics, these gemmules nniltiply and become esjiecially concen- 

 tiated in the germ cells; in the latter tlu'y unite with others like them- 

 selves; in course of development they grow into cells like those from 

 which they were oiiginally given off. (See Fig" 1, Diagram ii.) 



(ralton, who has always been doubtful in regard to use inheritance, 

 while advancing a theory of "continuity," partly approved Darwin's 

 pangeiH^sis idea in the cautious statement: "Each c<'ll may throw oft' a 

 few germs that find their way into the circulatif)n and thereby have a 

 chance of entering the germ cells." j At the same time (lalton contrib- 

 uted very important experimental disproof of the existence of "gem- 

 mules," and in fact — of the popular ideaof the circulation of hereditary 

 characters in the blood, by a series of careful experiments upon the 



*See Proc, Jioij. Soc. Edin., 1888, p. ,0,3. 



t Seo .tnimaJx and I'laiil-s itiidcr [)<)nu'n1icnl'uiii .Xf'l-u \<»1. ii, p. .'{"iy. 



XCoiittiiipornrji li'vviviv, vol. \.\\ii. p. .S()-J)5. 



