iQ2oJ CURRENT LITERATURE 531 



The results involved increase in size (number as well as size of cells), weight, 

 yield, early flowering, longevity, resistance to climate and disease, seed viabil- 

 ity, and ease of vegetative propagation. Foreign pollen immediately produces 

 larger endosperm (maize) than does own pollen in different seeds of the same 

 ear. 2 No selective fertilization in favor of foreign pollen is associated with 

 hybrid vigor. 3 Those characters which are quickest to be modified by exter- 

 nal factors also show the greatest degree of hybrid vigor on crossing. On 

 hybrid vigor in animals, which is less noticeable, the work of Castle is 

 described, and that of some others briefly mentioned. Chapter viii sketches 

 in historical sequence the theoretical mechanisms which have been provided 

 to explain hybrid vigor, culminating with the junior author's explanation 

 through "dominance of linked factors. " 4 The faint possibility of fixing 

 u hybrid" vigor is discussed. Chapter ix discusses the relation of the problem 

 to sterility. Two distinct types of sterility occur: (1) inbreeding may isolate 

 sterile strains in the same way that it isolates other characters; and (2) rather 

 wide crosses may produce a sterile Y l because through the degree of difference 

 between the uniting germ plasms " the precise and complex machinery govern- 

 ing gametogenesis cannot do its work in the normal manner, and sterility 

 results, although under the same conditions developmental (somatic) cell 

 division goes on as usual." 



Chapter x sketches the role of inbreeding and outbreeding in evolution. 

 It contains a number of interesting speculations, among which the most 

 striking are that hybrid vigor may be pictured as the efficient cause of the 

 establishment of the sex habit itself, and of the rise of the sporophyte genera- 

 tion in plants. The authors differ from many geneticists in claiming that 

 "bud variations occur much more frequently in heterozygotes than in homozy- 

 gotes." Chapter xi outlines the value of inbreeding and outbreeding in 

 plant and animal improvements. "There must be cross-breeding to furnish a 

 variety of character combinations from which to select; there must be inbreed- 

 ing to isolate the combinations desired." The practical utilization of hybrid 

 vigor is pictured also, a fairly common practice in live stock breeding, a rare 



* 



one in plants other than maize, for which the authors commend a double 

 crossing system. 



The two concluding chapters present applications to the human race. 

 Although avowedly less exact than the earlier chapters, these are in a sense 

 the most interesting of all, as may appear in the following summarizing quo- 

 tations. 



"Owing to the existence of serious recessive traits there is objection to 

 indiscriminate, irrational, intensive inbreeding in maft; yet inbreeding is the 

 surest means of establishing families which as a whole are of high value to 



fertilization. Bot. Gaz. 65 '.324-333 



figs. 3. 1918. 



* Box. Gaz. 68:150. 1919. « Bot. Gaz. 66:70. 1918. 



