388 CARNEGIE INSTITUTION OF WASHINGTON. 



It is becoming clear that inherited characteristics fall into three 

 categories: (1) those which sharply mendelize; (2) those which blend 

 without indication of Mendelian inheritance; (3) those which give 

 partial blending with imperfect Mendelian segregation. A satisfactory 

 explanation of the first sort is found in the assumed existence of 

 pecuUar chemical substances influencing development, which are 

 localized in particular chromosomes within the germ-cells. Nothing 

 is known as to the mechanism of transmission of blending characters. 

 As to the third sort, they may be supposed to depend partly upon sub- 

 stances localized in particular chromosomes, partly upon substances 

 not so localized, or it may be assumed that homologous chromosomes 

 which are dissunilar gradually lose their dissimilarity during conjuga- 

 tion. Investigation is needed to clear up these uncertainties. 



The efficacy of selection, whether natural or artificial, in evolution, 

 is generally conceded, but doubt still exists as to the nature and 

 course of the variations on which selection may act. That they are 

 directly and adaptively produced by the environment finds small 

 support in existing observational or experimental evidence. That a 

 changing environment may induce variation by disturbing the 

 stabiUty of the chemical compounds which constitute the Uving 

 substance and so leading them to assume new forms seems probable, 

 but as yet lacks verification. The one certain generalization of 

 genetic research is that hybridization is a potent source of variation. 

 But hybridization consists essentially in bringing together into a 

 single organism two somewhat dissimilar kinds of living substance, 

 from the interactions of which new sorts may arise slightly different 

 from either of the antecedent sorts. A further study of hybridiza- 

 tion and its consequences seems, therefore, the best present means 

 of extending our knowledge of organic evolution. 



