publi<^hed in 1838 and 1849. He divided his hybrids into 

 intermediate types, commingled types, and decided types. 



He noted, moreover, various forms in hybrids of the 

 second generation, but he did not see segregation He 

 noted also the increased vigor of growth of many hybrids ot 

 species. Gaertner's crossings were made upon species as 

 units and not upon the basis of unit-characters. 



Of the many problems of hybrids and hybridization 

 that presented themselves to Darwin, some have been solv- 

 ed by the aid of Mendel's Laws, but some still await solu- 

 tion. Among these may be mentioned : the cause of steril- 

 ity; why the hybrids of reciprocal crosses sometimes differ 

 in fertility; why some forms profit by crossing while others 

 do not; why a change of environment affects the sterility of 

 self-sterile species. 



It is worth while noting some of Darwin's observations 

 which come very near to those expressed by Mendel. He 

 says;" There are certain hybrids which instead of having, as 

 is usual, an intermediate character between the two parents, 

 always closely resemble one of them" {Origin of Species). 

 In crossing the normal snapdragon with the abnormal he 

 found all the hybrids normal, but when these hybrids were 

 selfed he obtained normal and abnormal forms in the ratio 

 of 88 to 37 — a close approximation to Mendelian expecta- 

 tions. 



He recognized latent characters, and explained "re- 

 version" as due to the sudden reappearance of some latent 

 ancestral character. He recognized segregation in the 

 second generation, for he says: "The grandchildren and suc- 

 ceeding generations (of crosses) continually revert in a 

 greater or lesser degree to one of both of their progenitors 

 {Variation of Plants and Animals under Domestication). 



Darwin maintained that the benefits from cross-fertili- 

 zation depend on the fact that plants taking part in the 

 cross have become differentiated by exposure to different 

 conditions. 



He also drew attention to the fact of immediate im- 

 provement due to a cross. He says: "After plants have 

 been propagated by self-fertilization for several generations, 

 a single cross with a fresh stock restores their pristine 

 vigor" {Effects of Cross and Self Fertilization in the Vege- 

 table Kingdom). 



Darwin's Theory as to the combination of characters 

 in hybridization is stated as follows: — " When two hybrids 

 pair, the combination of pure gemmules derived from the one 



76 



