12 INTRODUC TION 



variations, i.e. such as appear at birth or are developed " spontane- 

 ously," without discoverable connection with the activities of the 

 organism itself or the direct effect of the environment upon it, though 

 Darwin clearly recognized the fact that even such variations must 

 indirectly be due to changed conditions acting upon the parental 

 organism or on the germ. In a second class of variations were 

 placed the so-called acquired characters, i.e. definite effects directly 

 produced in the course of the individual life as the result of use and 

 disuse, or of food, climate, and the like. The inheritance of congen- 

 ital characters is now universally admitted, but it is otherwise with 

 acquired characters. The inheritance of the latter, now the most 

 debated question of biology, had been taken for granted by Lamarck 

 a half-century before Darwin ; but he made no attempt to show how 

 such transmission is possible. Darwin, on the other hand, squarely 

 faced the physiological requirements of the problem, recognizing that 

 the transmission of acquired characters can only be possible under the 

 assumption that the germ-cell definitely reacts to all other cells of the 

 body in such wise as to register the changes taking place in them. In 

 his ingenious and carefully elaborated theory of pangenesis, 1 Darwin 

 framed a provisional physiological hypothesis of inheritance in ac- 

 cordance with this assumption, suggesting that the germ-cells are 

 reservoirs of minute germs or gemmules derived from every part of 

 the body ; and on this basis he endeavoured to explain the trans- 

 mission both of acquired and of congenital variations, reviewing the 

 facts of variation and inheritance with wonderful skill, and building 

 up a theory which, although it forms the most speculative and hypo- 

 thetical portion of his writings, must always be reckoned one of his 

 most interesting contributions to science. 



In the form advocated by Darwin the theory of pangenesis has 

 been generally abandoned in spite of the ingenious attempt to remodel 

 it made by Brooks in i883. 2 In the same year the whole aspect of 

 the problem was changed, and a new'period of discussion inaugurated 

 by Weismann, who put forth a bold challenge of the entire Lamarckian 

 principle. 3 " I do not propose to treat of the whole problem of hered- 

 ity, but only of a certain aspect of it, the transmission of acquired 

 characters, which has been hitherto assumed to occur. In taking this 

 course I may say that it was impossible to avoid going back to the 

 foundation of all phenomena of heredity, and to determine the sub- 

 stance with which they must be connected. In my opinion this can 

 only be the substance of the germ-cells ; and this substance trans- 



1 Variation of Animals and Plants, Chapter XXVII. 



2 The Law of Heredity, Baltimore, 1883. 



3 Ueber Vererbung, 1883. See Essays upon Heredity, I., by A. Weismann, Clarendon 

 Press, Oxford, 1889. 



