416 



SCIENCE 



[N. S. Vol. LI. No. 1321 



that for permanent progTess not only experi- 

 ments are required but that the experiments 

 should be of a quantitative character. It will 

 be the purpose of this series of monographs to 

 emphasize and further as much as possible 

 this development of Biology." Until quite 

 recently discussions of inbreeding, whether by 

 biologists or others, have savored of anything 

 but the " methods of the exact sciences." It 

 is safe to say that no phase of biology has 

 been enveloped in such a fog of superstition, 

 old wives tales, and other sorts of misappre- 

 hension as has inbreeding. The investiga- 

 tions of East during the past decade and more 

 have been a potent and pioneer influence in 

 dissipating this fog. It is particularly appro- 

 priate that he and his former student Jones 

 should prepare a critical general review of the 

 really scientific work which has been done in 

 tliis field. It is a service which puts all 

 biologists considerably in their debt. 



After an introductory chapter^ which defines 

 the problem of inbreeding and shows its rela- 

 tion to practical questions of sociology and 

 agriculture, as well as biology, three chapters 

 are devoted to the statement of some ele-- 

 mentary biological facts and principles which 

 are essential to any rational discussion of a 

 problem which involves and arises out of the 

 phenomena of reproduction on the one hand, 

 and of heredity on the other hand. These 

 chapters, as would be expected by any one ac- 

 quainted with the authors' other writings^ are 

 models of clear and condensed exposition. 

 Chapter V. deals with " Mathematical Con- 

 siderations of Inbreeding " in which is re- 

 viewed recent work on the measurement of 

 the degree of inbreeding existent in complex 

 pedigrees, and on the gametic consequences 

 which must follow the continued inbreeding 

 of a Mendelian population. The analysis of 

 the latter jKjint shows that the amoimt or 

 degree of heterozygosity decreases with con- 

 tinued inbreeding. The authors state the ex- 

 pectations in the following words : 



Assurminig, then, that the loss of the stimulation 

 aecompanying heterozygosity is correlated with the 

 reduction in the number of heterozygous factors, Tve 

 should expe<it to find the decrease of heterosis 



greatest in the first generations, rapidly becoming 

 less until no further loss is noticeable in any num- 

 ber of subsequent generations of self-fertilization, 

 and that on the average the decrease will become 

 negligible from the seventh to the twelfth genera- 

 tion and from then on no fiirther marked change 

 will take place. Segregation of characters and ap- 

 pearance of new types and reduction in variability 

 will also follow the same course. Some cases are 

 to be expected in which stalbUity is reached earlier, 

 and some cases in which it is reached later; or, 

 theoretically it may never be reached. 



The next chapter reviews the actual results 

 of long continued inbreeding. The classic 

 data here are afforded, on the animal side, by 

 Miss King's brilliant experiments with the 

 white rat, and on the plant side by the no less 

 outstanding work of East and Jones on maize, 

 corroborated by the concordant but less ex- 

 tensive researches of Shull on the same form. 

 These two great experimental investigations 

 may fairly be regarded as a real triumph of 

 American biology. Operating in a field on 

 which a mass of inconclusive experimentation 

 and uncritical speculation had been carried out 

 these researches of East, Shull and Miss King 

 have essentially solved for all time the im- 

 portant features of the problem of inbreeding. 

 We now understand where formerly we specu- 

 lated. The main aspects of the problem are 

 now matters of exposition not debate. The 

 net result must be stated in the authors' 

 words: 



In tracing the evolution of ideas concerning the 

 effects of inbreeding and outbreeding we must give 

 great credit to Darwin for calling attention to the 

 importance of the phenomena in relation to evolu- 

 tion and for being the first to see that heredity dif- 

 ferences, rather than the mere act of crossing, was 

 the real point involved; but with all due credit to 

 Darwin, it was not until Mendelism became known, 

 appreciated, and applied that the first real attack 

 upon the problem was made possible. When linked 

 with Mendelian phenomena it was clearly recog- 

 nized for the first time that one and the same prin- 

 ciple was involved in the effects of inbreeding and 

 the directly opposite effects of outbreeding. In- 

 breeding was not a process of continual degenera- 

 tion. Injurious effects, if present, were due to the 

 eegregation of characters. In addition to this 

 segregation of characters the fact was established 



