Castle: Genetics and Eugenics 



71 



Figure 1 7 is a photograph of one of the 

 F2 litters most extreme in its range of 

 variation in growthiness. These four 

 individuals had received identical treat- 

 ment and none of them had ever been 

 noticed to be sick, yet when this picture 

 was taken when they were being mar- 

 keted at the age of eleven months the 

 weights were, from left to right: 221, 

 293, 90, and 170 lbs.! Possibly more 

 animals are born runts than is com- 

 monly believed. 



CONCLUSION 



It is not believed that all the hypo- 

 theses which have been tentatively ad- 

 vanced in this article will stand in 

 every detail without amendment the 

 test of further research. However 

 some of them will and if the others 

 awaken enough interest to stimulate 



further research with this economically 

 important animal, they will have 

 served a useful purpose. 



POSTSCRIPT 



Since the above was written a 

 report^ of the results of a cross between 

 mule-foot and Duroc-Jersey swine has 

 been published which agrees closely 

 with two of the main conclusions 

 reached in this experiment: Namely, 

 that the presence of black as contrasted 

 to its absence is dependent upon a 

 single factor and that the different 

 shades of red are due to the interaction 

 of a few independent factors which do 

 not affect black. The black of the 

 mule-foot swine behaves like that of 

 the American Hampshire but whether 

 the two are genetically identical is 

 still undetermined. 



Castle's Book Revised 



Genetics and Eugenics: A Text- 

 book for Students of Biology and a 

 Reference Book for Ani^mal and 

 Plant Breeders, by W. E. Castle, 

 Professor of Zoology in Harvard 

 University and Research Associate 

 of the Carnegie Institution of Wash- 

 ington. Harvard University Press 

 1920. 395 pages, 154 figures. 



Its clearness and pleasing style as 

 well as its thoroughness of treatment 

 have made Professor Castle's "Genetics 

 and Eugenics" one of the most exten- 

 sively used books on its subject since 

 its appearance in 1916. In the second 

 edition, which has recently appeared, 

 several chapters have been rewritten 

 and the total length has been increased 

 by 42 pages in order to take account 

 of the rapid progress of our knowledge. 

 The subjects of linkage and the nature 

 of genetic change are dealt with at 

 considerably greater length. 



The greatest change in viewpoint 

 is with respect to the hereditary basis 

 of fluctuating variation. In his earlier 

 work. Professor Castle assumed that 

 adult characteristics depend on rela- 



' Detlefsen, J. A. and Carmichael, W. J. Inheritance of Syndactylism, Black, and Dilution 

 in Swine. Journal of Agricultural Research, Vol. 20, pp. 595-604. 



tively few mendelizing unit factors, 

 and accounted for his success in bring- 

 ing about great changes, by means of 

 long continued selection of minute 

 variations, by holding that the heredi- 

 tary factors are themselves subject 

 to fluctuating variation. His own 

 experiments, however, as well as others, 

 stimulated to a large extent by his 

 championship of the above view, have 

 now demonstrated beyond question 

 the remarkably high degree of con- 

 stancy of most Mendelian factors, and 

 have made it clear that each character- 

 istic is in general affected by many 

 factors. Fluctuating variation is thus 

 interpreted in the present edition as 

 depending largely on the recombina- 

 tions possible in a system of many unit 

 factors, each insignificant in its own 

 effects, and to a much less extent upon 

 mutation of the units thciiiselves. 



Taken as a whole, the book can be 

 heartily recommended as giving a 

 reliable and very readable account of 

 the present state of knowledge of 

 genetics together with a sane discus- 

 sion of the application to eugenics. — ■ 

 S. W. 



