II.] ON HEREDITY. 83 



explain why the offspring should suffer from the same disease 

 as that which was artificially induced in the parents. But this 

 does not appear to have been by 2iX\y means invariably the 

 case. Brown-Sequard himself says, 'The changes in the eye 

 of the offspring were of a very variable nature, and were only 

 occasionally exactly similar to those observed in the parents.' 



There is no doubt, however, that these experiments demand 

 careful consideration, but before they can claim scientific re- 

 cognition, they must be subjected to rigid criticism as to the 

 precautions taken, the number and nature of the control ex- 

 periments, etc. 



Up to the present time such necessary conditions have not 

 been sufficiently observed. The recent experiments themselves 

 are only described in short preliminary notices, which, as re- 

 gards their accuracy, the possibility of mistake, the precautions 

 taken, and the exact succession of individuals affected, afford 

 no data upon which a scientific opinion can be founded. Until 

 the publication of a complete series of experiments, we must 

 say with Du Bois Reymond ^, ' The hereditary transmission of 

 acquired characters remains an unintelligible hypothesis, which 

 is only deduced from the facts which it attempts to explain.' 



We therefore naturally ask whether the hypothesis is really 

 necessary for the explanation of known facts. 



At the first sight it certainly seems to be necessary, and it 

 appears rash to attempt to dispense with its aid. Man}^ pheno- 

 mena only appear to be intelligible if we assume the hereditary 

 transmission of such acquired characters as the changes which 

 we ascribe to the use or disuse of particular organs, or to the 

 direct influence of climate. Furthermore, how can we explain 

 instinct as hereditary habit unless it has gradually arisen by 

 the accumulation, through heredity, of habits which were prac- 

 tised in succeeding generations ? 



I will now attempt to prove that even these cases, so far as 

 they depend upon clear and indubitable facts, do not force us 

 to accept the supposition of the transmission of acquired 

 characters. 



It seems difficult and well nigh impossible to deny the trans- 

 mission of acquired characters when we remember the influence 

 which use and disuse have exercised upon certain special 



^ See ' Ueber die Uebung,' Berlin, 1881. 

 G 2 



