332 Evolution and Adaptation 



appear at once, and their results last only as long as the 

 influences themselves last, and are then lost, leaving nothing 

 permanent behind. This is true even when the external 

 influences have lasted for a long time, — since the glacial 

 epoch, for instance. We find, he claims, nothing that sup- 

 ports the view that such influences are inherited. 



If we next examine the question of changes from internal 

 causes, Nageli claims that here also observation and research 

 fail to show the origin of a new species, or even of a new 

 variety from external causes. In the organic world little 

 change has taken place, he believes, since the glacial epoch. 

 Many varieties have even remained the same throughout the 

 whole intervening time ; and while it cannot be doubted that 

 new varieties have also been formed, yet the cause of their 

 origin cannot be empirically demonstrated. The permanent, 

 hereditary characters, of whose origin we know something 

 from experience, belong to the individual changes which 

 have appeared under cultivation in the formation of domestic 

 races. These are for the most part the result of crossing. 

 So far as we have any definite information as to the origin of 

 the changes, they are the result of inner, and never of exter- 

 nal, causes. W_ e recogniz e that this must be the case, since 

 under the same external conditions individuals behave differ- 

 ently — in the same flower-bud some seeds give rise to plants 

 like the parent, others to altered ones. The strawberry with a 

 single leaflet, instead of three, arose in the last century in a 

 single individual amongst many other ordinary plants. From 

 the ten seeds of a pear Van Mons obtained as many different 

 kinds of pears. The most conclusive proof of the action 

 of inner causes is most clearly seen when the branches of 

 the same plant differ. In Geneva a horse-chestnut bore 

 a branch with " filled " flowers, and from this branch, by 

 means of cuttings, this variation has been carried over all 

 Europe. In the Botanic Garden at Munich there is a beech 

 with small divided leaves ; but one of its branches produces 



