lO INTRODUCTION 



acters ; i.e. changes that arise in the course of the individual Ufe as 

 the effect of use and disuse, or of food, cHmate, and the Hke. The 

 inheritance of congenital characters is now universally admitted, but 

 it is otherwise v^'ith 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 prob- 

 lem, 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 elab- 

 orated theory of pangenesis, ^ Darwin framed a provisional physio- 

 logical hypothesis of inheritance in accordance 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 transmission both of acquired and of con- 

 genital variations, reviewing the facts of variation and inheritance 

 with wonderful skill, and building up a theory which, although it forms 

 the most speculative and hypothetical portion of his writings, must 

 always be reckoned one of his most interesting contributions to science. 

 The theory of pangenesis has been generally abandoned in spite 

 of the ingenious attempt to remodel it made by Brooks in 1883.^ 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.'^ "I do not 

 propose to treat of the whole problem of heredity, 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 substance with which 

 they must be connected. In my opinion this can only be the sub- 

 stance of the germ-cells ; and this substance transfers its hereditary 

 tendencies from generation to generation, at first unchanged, and 

 always uninfluenced in any corresponding manner, by that which 

 happens during the life of the individual which bears it. If these 

 views be correct, all our ideas upon the transformation of species re- 

 quire thorough modification, for the whole principle of evolution by 

 means of exercise (use and disuse) as professed by Lamarck, and 

 accepted in some cases bv Darwin, entirely collapses " {I.e., p. 69). 



1 Variation of .Animals and Plants, Chapter XXVIl. 

 - The Im7u of Heredity, Baltimore, 18S3. 



3 Ueber Vcrerhiin:^, 1S83. See Essays jipon Heredity, I., by A. Weismann, C'laremloii 

 Press, Oxford, 1S89. 



