502 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the sexual elements. Hence, it is not the reproductive organs or buds which 

 generate new organisms, but the units of which each individual is composed. 

 These assumptions constitute the provisional hypothesis which I have called 

 Pangenesis." 



Since the cells of the body are represented by gem mules within the germ- 

 cells, Darwin's theory is a theory of Preformation. It explains the facts of 

 the regeneration of lost parts by the assumption that the gem mules of the part 

 in question are disseminated throughout the body and have only to unite with 

 the nascent cells at the point of new growth. Pangenesis explains reversion, 

 since gemmules may lie dormant in one generation and develop in the next. 

 It explains congenital variation, since the mixture of maternal and paternal 

 gemmules is plainly different from the two kinds taken separately. It explains 

 how acquired variations may become congenital, since an altered part throws 

 oif altered gemmules, and by the collocation of these in the germ-cells the 

 alteration may be transmitted. It thus allows the transmission of acquired 

 characters. 



Darwin's assumptions of gemmules and their behavior are pure assump- 

 tions, for which subsequent investigation has not provided a basis of facts. 

 As we have seen, also, the inheritance of acquired characters is greatly in 

 doubt, and, if they are heritable at all, they can be so only comparatively 

 feebly. Besides these objections it was early found that, with the increase 

 of knowledge of the facts of heredity, it was necessary to modify very mate- 

 rially the theory of Pangenesis. This has been ably done successively by 

 Gal ton, 1 Brooks, 2 and de Vries. 3 But neither the original theory nor its 

 modifications have been generally accepted. 



Wei&mann's Theory. — Since 1880, Professor Weismann*of Freiburg has 

 published numerous essays upon heredity and allied subjects, in which, besides 

 reviewing the views of others, he has developed in detail a new and elaborate 

 theory of his own, that is the most ambitious attempt yet made to solve the 

 problem of inheritance. In the course of their development Weismann's 

 ideas have undergone some modification. Their leading features are as 

 follows : 



The essential hereditary substance, or germ-plasm, is the chromatin of the 

 nucleus of the germ-cells. One of the fundamental tenets of Weismann's 

 system is expressed by his own phrase, "the continuity of germ-plasm." By 

 this is meant that the germ-plasm of one individual, instead of arising de novo 

 in the individual by the collocation of multitudinous "gemmules" derived 

 from the body-cells, originates directly from the germ-plasm of the parent, 

 thence from that of the grandparent, and so on backward through all genera- 

 tions to the origin of all germ-plasms that took place simultaneously with the 



1 Francie I ralton : "A Theory of Heredity," Journal of the Anthropological Institute, 1875. 



2 \V. K. Brooks: The. Laws of Heredity, 1883. 



8 II. de Vries: Die Intra cellular e Pangenesis, 1889. 



* August Weismann : /■,'.- < ( y upon Heredity and Kindred Biological Problems, authorized 

 translation, vol. i., 1889; vol. ii., L892; The Germ-plasm, authorized translation, 1893; The 

 Effect of External Injlncnccs upon Dccclopuinit, the Romanes Lecture, lS'.M. 



