124 THE AMERICAN NATURALIST [Vol. XL VI 



to understand how new characters could ever appear 

 except under one or the other of these conditions. We 

 particularly need at this time more knowledge of the 

 mechanism underlying the gross phenomena of Men- 

 delian inheritance, and then perhaps we may learn under 

 what conditions this mechanism may be altered. 



As a result of the work of Mendel and his followers we 

 know much more about heredity than was known before, 

 we have learned how to separate and to combine hered- 

 itary characters, we have learned to look for evolution in 

 the appearance of new characters, but we have not learned 

 how to produce new characters. 



II 



Practically all who have ever thought or written on 

 evolution have found the principal causes of the trans- 

 mutation of old characters into new ones in the action of 

 extrinsic, or environmental, forces on the organism. As 

 the result of years of labor on this subject Darwin con- 

 cluded that * i variability of every sort is due to changed 

 conditions of life." It is well known that environmental 

 changes produce many kinds of modifications in organ- 

 isms, and in general these modifications are the more pro- 

 found the earlier they occur in ontogeny; it is known that 

 slight alterations of the germ cells may produce great 

 modifications of adult structure, and it seems reasonable 

 to suppose that environmental changes of the right sort 

 applied to the germ cells at the right stage would lead to 

 a permanent modification of the substance of heredity 

 and hence to the appearance of new characters of evolu- 

 tionary value. And yet one of the most striking results 

 of recent work is to show the small effect of environ- 

 mental changes of all sorts on racial characters. Marked 

 individual modifications may be produced which do not 

 become racial. Usually not one of thousands of varia- 

 tions which occur have any evolutionary value. These 

 variations come with changing environment and with 

 changing environment they disappear. Just as in the 

 individual, so also in the race there seems to be a power 



