II.] ON HEREDITY. 8 1 



The difficulty or the impossibihty of rendering the trans- 

 mission of acquired characters intelhgible by an appeal to any 

 known force has been often felt, but no one has hitherto at- 

 tempted to cast doubts upon the very existence of such a form 

 of heredity. 



There are two reasons for this : first, observations have been 

 recorded which appear to prove the existence of such trans- 

 mission ; and secondly, it has seemed impossible to do without 

 the supposition of the transmission of acquired characters, 

 because it has always played such an important part in the 

 explanation of the transformation of species. 



It is perfectly right to defer an explanation, and to hesitate 

 before we declare a supposed phenomenon to be impossible, 

 because we are unable to refer it to any of the known forces. 

 No one can believe that we are acquainted with all the forces of 

 nature. But, on the other hand, we must use the greatest 

 caution in dealing with unknown forces ; and clear and in- 

 dubitable facts must be brought forward to prove that the 

 supposed phenomena have a real existence, and that their 

 acceptance is unavoidable. 



It has never been proved that acquired characters are trans- 

 mitted, and it has never been demonstrated that, without the 

 aid of such transmission, the evolution of the organic world be- 

 comes unintelligible. 



The inheritance of acquired characters has never been proved, 

 either by means of direct observation or by experiment'. It 

 must be admitted that there are in existence numerous descrip- 

 tions of cases which tend to prove that such mutilations as the 



clear that such impulses do not originate in the constitution of the tissue 

 in question, but are due to the operation of external causes. The 

 activity does not arise directly from any natural disposition of the germ, 

 but is the result of accidental external impressions. A domesticated duck 

 uses its legs in a different manner from, and more frequently than a wild 

 duck, but such functional changes are the consequence of changed ex- 

 ternal conditions, and are not due to the constitution of the germ. 



^ Upon this subject Pfluger states—' I have made myself accurately 

 acquainted with all facts which are supposed to prove the inheritance of 

 acquired characters,— that is of characters which are not due to the 

 peculiar organization of the ovum and spermatozoon from which the 

 individual is formed, but which follow from the incidence of accidental 

 external influences upon the organism at any time in its life. Not one 

 of these facts can be accepted as a proof of the transmission of acquired 

 characters.' /. c. p. 68. 



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