November ii, 1920] 



NATURE 



335 



Biology of Endogamy and Exogamy. 



Inbreeding and Outbreeding: Their Genetic and 

 Sociological Significance. By Dr. E. M. East 

 and Dr. D. F. Jones. (Monographs on Experi- 

 mental Biology.) Pp. 285. (Philadelphia and 

 London : J. B. Lippincott Co., 1919.) Price 

 I05. 6d. net. 



IT has been shown that close inbreeding of good 

 stock, if associated with the usual common- 

 sense elimination of wasters, may be persisted in 

 for several generations without any undesirable 

 consequences. Many fine breeds of animals and 

 races of plants have had very close inbreeding at 

 their beginnings. It is said that there is habitual 

 endogamy among bees and ants, and we know 

 that in some formicaries the females are insemin- 

 ated without an outside excursion. It seems then 

 that "inbreeding is not in itself harmful." This 

 is the first conclusion that the authors of this 

 excellent monograph reach. 



One can go further, however, and maintain 

 that close mating is positively advantageous. It 

 fixes desirable characters and leads towards the 

 establishment of a uniform and stable herd. But 

 it is well known that this is only half the truth ; 

 there is sometimes an advance, but there is often 

 a disappointing regression — a reduction of 

 vigour, resisting power, fecundity, and even size. 

 Thus the authors tell us that in the case of grain 

 there is a reduction of productivity to perhaps 

 half of what it was, while in the case of maize 

 there is a marked reduction in size and rate of 

 growth. The problem then is to explain the de- 

 terioration that sometimes sets in, and to discover 

 whether it is due to the consanguinity as such or 

 to something else. 



Drs. East and Jones point out that if there 

 are, to begin with, in the inheritance of the herd, 

 say, four distinct hereditary factors relative to a 

 particular character, such as the colour of the 

 pellage, the automatic effect of the inbreeding will 

 be to isolate four types, each pure as regards the 

 particular character. But some of the characters 

 which arc thus segregated may be undesirable 

 " recessivcs," seldom seen in ordinary circum- 

 stances, because they are hidden by their "domin- 

 .int" allelomorphs. "These recessivcs are the 

 ' corrupt fruit ' which give a bad name to in- 

 ceding, for they are often — very often — un- 

 desirable characteristics." "Inbreeding tore aside 

 the mask, and the undesirable characters were 

 shown up in all their weakness, to stand or fall 

 on their own merits." This is an interesting 

 theoretical interpretation, and it immediately sug- 

 NO. 2663, VOL. 106] 



gests a practical application. For if the inbreed- 

 ing brings to the surface certain characters which 

 were in the general inheritance of the stock, but 

 were kept out of sight by more favourable char- 

 acters which dominated them, it is open to the 

 breeder to practise stern elimination, to get rid of 

 the " isolated " types with Undesirable characters, 

 leaving the stock all the better for its purgation. 

 It need scarcely be said that the authors back up 

 their conclusion with a wealth of experimental 

 data, and that they give it the fit and proper 

 technical formulation. 



It is often useful to stand back, as it were, 

 from the realm of organisms and the age-long 

 process of organic evolution, and ask ourselves 

 what the great steps or trends have been. We, 

 mean such steps as getting out of the water, sub- 

 stituting sexual for asexual reproduction, and 

 establishing viviparity. Another great trend is 

 the securing of cross-fertilisation, though the 

 range of the cross varies within wide limits. 

 Parthenogenesis has been tried, and it seems to 

 work well enough among Rotifers; autogamy or 

 self-fertilisation has been tried, and it seems to 

 work well enough in the liver-fluke ; but the broad 

 fact is that some form of cross-fertilisation is the 

 rule. And if we ask for the. deep reason justify- 

 ing this, the probable answer is that the survival 

 value of cross-fertilisation lies largely in the fact 

 that it promotes variability. It brings about a 

 greater variety of raw material on which selective 

 agencies can operate. Now the authors bring 

 forward experimental evidence to show that the 

 wider ranges of cross-fertilisation which are 

 called outbreeding are valuable in promoting 

 variability, and they have an interesting dis- 

 cussion of the limits of profitable crossing. The 

 strains crossed may easily be too unlike. 



But there is another advantage in outbreeding, 

 that it promotes the mysterious quality called 

 "vigour." Darwin was strongly of opinion that 

 the gain in constitution derived from an occa- 

 sional crossing was a more important biological 

 fact than the loss that sometimes follows close 

 inbreeding; the authors confirm this shrewd judg- 

 ment. What is the meaning of this "hybrid 

 vigour," which may be associated with increase 

 in resisting power, in size, and in other good 

 qualities? It has been suggested that some 

 physiological stimulus comes to the offspring be- 

 cause of the unlikeness or apartness of the 

 parents, but that is not the authors' view. They 

 think that the reason for "hybrid vigour" is to 

 be found in the pooling of diver.se hereditary 

 resources of goo<l quality. The crossing makes 

 it more likely that a minus on one side may be 



