448 THE AMERICAN NATURALIST [VOL. XLII 



variation, and that in consequence of this fact it is im- 

 proper to apply principles derived under cultivation to 

 plants growing free in nature. There can be no doubt 

 that good cultural conditions render patent many internal 

 characters which are invisible under conditions of poor 

 nutrition and crowding, and this fact together with the 

 fact that many of the common culture-plants are complex 

 hybrids, may fully account for the general impression 

 regarding the effects of culture. There is no satisfactory 

 evidence that good feeding and other conditions usually 

 supplied under tillage have any effect in the production 

 of the mutations upon which the external characters no 

 doubt ultimately depend. 



GENERAL CONSIDERATIONS 



It is obvious from the foregoing results and discussions 

 that latency is not a simple phenomenon, but may be due 

 to a number of different circumstances. The point which 

 I have strongly emphasized in my two preceding papers 

 on the subject of latency namely, that cases of latency 

 must be explained, not upon the ground of inactivity or 

 dormancy of characteristics, but simply on their invisi- 

 bility is fully borne out by all the facts here presented. 

 The several different types depend upon the different 

 causes for the invisibility of the characteristics. 



Of the four types of latency here recognized, the first 

 three types those in which latency is due to definite in- 

 terrelations between Mendelian units will give rise to 

 definite characteristic ratios which are as constant for 

 each case as the typical ratios are for typical Mendelian 

 phenomena. This is not so with latency due to fluctua- 

 tion, as the variable conditions upon which the fluctua- 

 tions depend may be such that any proportion of the indi- 

 viduals from none to all may have the character in ques- 

 tion latent. This is not only true of the characters of 

 pure-bred types as exemplified by Bursa bursa-pastoris, 

 but is even more apt to be true of heterozygotes, thus re- 

 sulting in many deviations from the correct ratios, as 



