1 84 TRANSMISSION OF ACQUIRED CHARACTERS 



of the leg more in proportion to the whole skeleton than do the 

 same bones in the wild duck ; in cows and goats which are habitually 

 milked the udders are large ; moles and many cave-animals have 

 rudimentary eyes. Cases like these may be in part regarded as 

 instances of individually re-acquired modifications, but they are 

 for the most part readily interpreted as due to the selection of 

 germinal variations. 



Misunderstanding IY Mistaking the reappearance of a 

 modification for transmission of a modification. It is of little 

 service to cite cases where a particular modification reappears 

 generation after generation unless it be shown that the change 

 recurs as part of the inheritance, and not simply because the 

 external conditions which evoked it in the first generation still 

 persisted to evoke it in those that followed. Reappearance 

 is not synonymous with inheritance. 



Illustration. When Prof. Nageli brought Alpine plants (Hieracium, 

 etc.) to the Botanical Garden at Munich, many became in the 

 first year so much changed that they were hardly recognisable 

 as the same species, and their descendants in the garden were 

 likewise quite different from their Alpine ancestors. The small 

 Alpine hawkweeds became large and thickly branching, and 

 blossomed freely. In some cases many generations were observed 

 even for thirteen years ; there was no doubt as to the reappearance 

 of the acquired characters ; but it was not thereby proved that the 

 reappearance was due to the inheritance. On the contrary, that 

 the reappearance was due to the persistence of the novel conditions, 

 to the changes which these directly impressed on each successive 

 crop, was shown by the fact that when the plants were removed to 

 poor, gravelly soil, the acquired characters disappeared, and the 

 plants were re-transformed into their original Alpine character. 

 " The re-transformation was always complete, even when the species 

 had been cultivated in rich garden soil for several generations." 



Misunderstanding Y Mistaking re-infection for transmission. 

 A particular form of the fourth misunderstanding has to do 

 with facts so special that it may be conveniently treated of 

 separately. It has to do with microbic diseases. It is ad- 



