ON HEREDITY. 81 



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 1 

 with unknown forces ; and clear and indubitable 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 becomes 

 unintelligible. 



The inheritance of acquired characters has never been proved, 

 either by means of direct observation or by experiment 1 . It must 

 be admitted that there are in existence numerous descriptions of 

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

 fingers, the scars of wounds, etc., are inherited by the offspring, 

 but in these descriptions the previous history is invariably obscure, 

 oand hence the evidence loses all scientific value* 



As a typical example of the scientific value of such cases I may 

 mention the frequently quoted instance of the cow, which lost its 

 left horn from suppuration, induced by some ' unknown cause,' and 

 which afterwards produced two calves with a rudimentary left horn 

 in each case. But as Hensen 2 has rightly remarked, the loss of 

 the cow's horn may have arisen from a congenital malformation, 

 which would certainly be transmitted, but which was not an ac- 

 quired character. 



The only cases worthy of scientific discussion are the well-known 

 experiments upon guinea-pigs, conducted by the French physiologist 

 Brown- S6quard. But the explanation of his results is, in my 

 opinion, open to discussion. In these cases we have to do with 

 the apparent transmission of artificially produced malformations. 

 The division of important nerves, or of the spinal cord, or the 



1 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 in- 

 cidence 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.' 1. c. p. 68. 



2 ' Physiologic der Zeugung.' 



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