600 CONTINUITY OF LIFE 



height, which, as we have seen, is inherited is famihar with the value of hybrid 



through multiple factors, led him to formu- corn, which demonstrates heterosis very 



late his filial regression law. This law markedly. Some striking cases of heterosis 



merely states that offspring of tall or short among human beings have occurred where 



parents, while taller or shorter, respectively, two distinctly different races have crossed, 



than the average British subject, are still The most celebrated instance is that of 



not as tall or as short as their parents. This Captain Christian and his fellow mutineers 



means that at the extremes of height off- of the "Bounty" who took native Polynesian 



spring tend to regress toward the average women from Tahiti and settled on tiny Pit- 



(FicT. 24-11). cairn Island. The first generation of chil- 



Although Galton's studies contributed dren were more vigorous, taller, and more 

 virtually nothing to the mechanics of in- fertile than either the white or the Polyne- 

 heritance, they did give later workers, not sian stock from which the parents came, 

 only in biology but also in sociology, eco- Incidentally, this is also one of the few 

 nomics, and other fields, a tool by which a cases in human history where a nearly per- 

 quantitative description of a naturally or feet inbreeding experiment was carried on. 

 artificially occurring series of variables Studies of half-breed Indians, as well as 

 could be stated. It has been used exten- hybrids from Hottentot and Boer crosses, 

 sively in biology in determining a normal have likewise revealed heterosis to a 

 population in respect to one or more traits, marked degree, although the condition does 

 This is important to animal breeders who not seem to be evident on certain black- 

 are trying to improve their stock with re- white crosses. 



spect to a particular character, such as egg The second generation following the 



laying in chickens. By carefully selecting original crosses usually results in a gradual 



for breeding stock, those birds that produce loss of the early hybrid vigor and the olf- 



the greatest number of eggs the whole pop- spring in succeeding generations slowly 



ulation can gradually be improved along return to an intermediate type. This might 



that line. This means a selection toward all be explained by considering that each 



dominant or all recessive genes. When the member of the first cross possesses certain 



stock becomes homozygous for a particular dominant genes for desirable traits which 



trait there is nothing more to be gained by were lacking in the mate. When crossed, 



further selection. One needs only to inbreed a now full complement of dominants would 



and keep the desired qualities which have produce their desirable effects, thus pro- 



thus been attained. ducing the vigorous offspring. In subse- 

 quent hybrid matings the dominant genes 



Hybrid vigor or heterosis would segregate out, as one might expect, 



One of the strange and unexplained out- and eventually the recessives would once 



comes of multiple factor inheritance is that more reduce most of the race to the condi- 



the hybrid resulting from two parents with tion of the original parental stock. 



contrasting traits is frequently more vigor- 



u- J 4-1 u 4-*. • \. , Inheritance of sex 

 ous, bigger, and apparently better m every 



way than either parent. This condition is The obvious fact that sexes appear in the 



known as heterosis or hybrid vigor. This ratio of 1 : 1 meant nothing until geneticists 



observation has been made on a great many noted that the cross of a hybrid ( Ww ) and 



animals. The mule, which is a result of a its double recessive (ww) was also a 1:1 



mating between a jack and a mare, is ratio, which fact suggested the possibility 



notorious for its strength and endurance that sex might likewise be determined ge- 



as well as its stubborness. Every farmer netically. Cytological evidence before 1900 



