l6(S CONTINUITY OF THE GERM-PLASM AS THE [IV. 



solution may be expected in the future. Neither Hackel's ' 

 ' Perigenesis of the Plastidule,' nor Darwin's^ 'Pangenesis,' 

 can be regarded as such a beginning. The former hypothesis 

 does not really treat of that part of the problem which is here 

 placed in the foreground, viz. the explanation of the fact that 

 the tendencies of heredity are present in single cells, but it is 

 rather concerned with the question as to the manner in which 

 it is possible to conceive the transmission of a certain tendency 

 of development into the sexual cell, and ultimately into the 

 organism arising from it. The same may be said of the hypo- 

 thesis of His^, who, like Hackel, regards heredit}'' as the trans- 

 mission of certain kinds of motion. On the other hand, it must 

 be conceded that Darwin's hypothesis goes to the very root of 

 the question, but he is content to give, as it were, a provisional 

 or purely formal solution, which, as he himself says, does not 

 claim to afibrd insight into the real phenomena, but only to 

 give us the opportunity'' of looking at all the facts of heredity 

 from a common standpoint. It has achieved this end, and I 

 believe it has unconsciously'' done more, in that the thoroughly 

 logical application of its principles has shown that the real 

 causes of heredity cannot lie in the formation of gemmules or 

 in any allied phenomena. The improbabilities to which any 

 such theory would lead are so great that we can affirm with 

 certainty that its details cannot accord with existing facts. 

 Furthermore, Brooks' ^ well-considered and brilliant attempt 

 to modify the theory of Pangenesis, cannot escape the reproach 

 that it is based upon possibilities, which one might certainly 

 describe as improbabilities. But although I am of opinion that 

 the whole foundation of the theory of Pangenesis, however it 

 may be modified, must be abandoned, I think, nevertheless, its 

 author deserves great credit, and that its production has been 

 one of those indirect roads along which science has been com- 

 pelled to travel in order to arrive at the truth. Pangenesis is 

 a modern revival of the oldest theory of heredity, that of De- 

 mocritus, according to which the sperm is secreted from all 



' Hackel, ' Ueber die Wellenzeugung der Lcbcnstheilclien, etc.,' 

 Berlin, 1876. 



^ Darwin, ' The Variation of Animals and Plants under Domestication,' 

 vol. ii. 1875. chap, xxvii. pp. 344-399. 



^ His, ' Unsre KOrperform, etc.,' Leipzig, 1875. 



* Brooks, 'The Law of Heredity,' Baltimore, 1883. 



