164 OUTLINES OF EVOLUTIONARY BIOLOGY 



cells of the body, so that when they develop they will give 

 rise to all those different kinds of cells arranged in the same 

 way as in the parent ? We must now briefly consider some 

 of the numerous attempts which have been made to answer 

 this question. 



Darwin, in 1868, put forward his theory of Pangenesis as a 

 provisional hypothesis to explain the facts of heredity, and this 

 theory, though it seems never to have met with any large 

 measure of acceptance, is of considerable historical interest. 1 

 He supposed that all the constituent cells of which the body is 

 composed not only multiply by ordinary cell-division, so as to 

 build up the various tissues, but also, throughout life, give off 

 extremely minute " genmmles " which wander tbrough the body 

 and are collected in vast numbers in the germ cells. The 

 gemmules, although so small as to be invisible even with the 

 highest powers of the microscope, are supposed to be capable of 

 absorbing nutriment and multiplying by division, and each one 

 is supposed, in some mysterious and unexplained manner, to 

 represent the particular cell of the body from which it was 

 derived, and to be capable, at the proper time and in the proper 

 place, of impressing the character of its parent cell upon a 

 corresponding cell of the new organism which develops from the 

 germ cell. 



In Darwin's own words : 



" The development of each being, including all the forms of 

 metamorphosis and metagenesis, depends on the presence of 

 gemmules thrown off at each period of life, and on their develop- 

 ment, at a corresponding period, in union with preceding cells. 

 Such cells may be said to be fertilized by the gemmules which 

 come next in due order of development. Thus the act of 

 ordinary impregnation and the development of each part in each 

 being are closely analogous processes. The child, strictly 

 speaking, does not grow into the man, but includes germs which 

 slowly and successively become developed and form the man. In 

 the child, as well as in the adult, each part generates the same 

 part. Inheritance must be looked upon as merely a form of 

 growth, like the self-division of a lowly organized unicellular 

 organism. Reversion depends on the transmission from the 

 forefather to his descendants of dormant gemmules, which 

 occasionally become developed under certain known or unknown 

 conditions. Each animal and plant may be compared with a bed 



1 For a complete exposition of the theory see Darwin's "Animals and Plants 

 under Domestication" (Ed. 2, Vol. II., Chapter XXVII.). 



