Essays on Life 



oppressive. But if the new be too foreign, 

 we cannot fuse the old and the new nature 

 seeming to hate equally too wide a deviation 

 from ordinary practice and none at all. This 

 fact reappears in heredity as the beneficial 

 effects of occasional crossing on the one hand, 

 and on the other, in the generally observed 

 sterility of hybrids. If heredity be an affair 

 of memory, how can an embryo, say of a mule, 

 be expected to build up a mule on the strength 

 of but two mule-memories ? Hybridism causes 

 a fault in the chain of memory, and it is to 

 this cause that the usual sterility of hybrids 

 must be referred. 



Fourthly, it requires many repeated impres- 

 sions to fix a method firmly, but when it has 

 been engrained into us we cease to have much 

 recollection of the manner in which it came 

 to be so, or indeed of any individual repetition, 

 but sometimes a single impression, if prolonged 

 as well as profound, produces a lasting impres- 

 sion and is liable to return with sudden force, 

 and then to go on returning to us at intervals. 

 As a general rule, however, abnormal impres- 

 sions cannot long hold their own against the 

 overwhelming preponderance of normal autho- 



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