433 DARWINISM chap. 



although it is very difficult to make them intelligible to persons 

 unfamiliar with the main facts of modern embryology. 1 



The problem is thus stated by Weismann : " How is it 

 that in the case of all higher animals and plants a single cell 

 is able to separate itself from amongst the millions of most 

 various kinds of which an organism is composed, and by 

 division and complicated differentiation to reconstruct a new 

 individual with marvellous likeness, unchanged in many cases 

 even throughout whole geological periods ? " Darwin at- 

 tempted to solve the problem by his theory of "Pangenesis," 

 which supposed that every individual cell in the body gave off 

 gemmules or germs capable of reproducing themselves, and that 

 portions of these germs of each of the almost infinite number of 

 cells permeate the whole body and become collected in the 

 generative cells, and are thus able to reproduce the whole 

 organism. This theory is felt to be so ponderously complex 

 and difficult that it has met with no general acceptance among 

 physiologists. 



The fact that the germ-cells do reproduce with wonderful 

 accuracy not only the general characters of the species, but 

 many of the individual characteristics of the parents or more 

 remote ancestors, and that this process is continued from 

 generation to generation, can be accounted for, Weismann 

 thinks, only on two suppositions which are physiologically 

 possible. Either the substance of the parent germ-cell, after 

 passing through a cycle of changes required for the construction 

 of a new individual, possesses the capability of producing anew 

 germ-cells identical with those from which that individual was 

 developed, or the new germ-cells arise, as far as their essential 

 and characteristic substance is concerned, not at all out of the body 

 of the individual, but direct from the parent germ-cell. This latter 

 view Weismann holds to be the correct one, and, on this theory, 

 heredity depends on the fact that a substance of special mole- 

 cular composition passes over from one generation to another. 

 This is the " germ-plasm," the power of which to develop itself 

 into a perfect organism depends on the extraordinary complica- 

 tion of its minutest structure. At every new birth a portion 



1 The outline here given is derived from two articles in Nature, vol. 

 xxxiii. p. 154, and vol. xxxiv. p. 629, in which Weismann's papers are summar- 

 ised and partly translated. 



