216 



THE AMERICAN NATURALIST 



[Vol. XLI 



"But the paternal and maternal representatives of a character 

 may in the meantime have exercised on each other a considerable 

 influence. In the case of some characters, as ear-length in rab- 

 bits (Castle, :05a), they completely blend and intermingle, so 

 that a new character is produced strictly intermediate between 

 the conditions found in the respective parents. 



''In other cases the modification may be slight, as if the pater- 

 nal and maternal representatives of a character had been scarcely 

 more than approximated. Sometimes in cases of alternative 

 inheritance no influence of the cross is observable in certain of 

 the 'extracted' individuals, but if any considerable number of 

 individuals is examined, others will be found in which the cross- 

 breeding manifests its influence. From this we conclude that 

 gametic purity is not absolute, even in sharply alternative inheri- 



These are very interesting qualifications of the Mendelian 

 principle of gametic purity. They suggest that new characters 

 might be swamped by repeated crossing, unless they were of 

 such overwhelming importance that they quickly won out in the 

 struggle for existence, to the immediate extinction of the bearers 

 of the older alternative characters. However, discussion may 

 here well wait upon further discovery. 



But this may be said: If characters are gradually in()(lifiaV)le, 

 time becomes a necessary element in experinu iiTs on evolution and 

 possibly long periods of time may be needed for the demonstra- 

 tion of certain slow natural processes. For the present we may 

 well hesitate to accept the conclusion that Mutation is the sole 

 and only possible mode of evolution. Refreshing as the new 

 method of research is, in the midst of oceans of tiresome specula- 

 tions, and most valuable and even absolutely indispensable as 

 the i-e>ults already arc, the latter arc certainly small compared 

 to the hulk of our iuii(»rancc rcH-arding inorphogenetic processes. 



and from infatuatio,. with ncw"i<lcas on the other, will look for 

 sonic iudci.cn.h'nl nutans of estimating the probable significance 

 of the new theories, it is the chief object of this paper to suggest 

 that such estimation may be rested upon the evidence of organic 

 geography when the evidence is available in sufficient body. 



