234 GENETICS AND EUGENICS 



and closely related to each other has on the whole the same 

 effect as self-fertilization. It brings together gametes which 

 transmit the same characters, which are doubtless chemi- 

 cally alike, and no particular increase of vigor results when 

 they unite. It is on a par with asexual reproduction by par- 

 thenogenesis, fission, budding, or vegetative multiplication. 

 There is in consequence no change in the germinal constitu- 

 tion, or relatively little. There is neither increase of vigor 

 nor loss of vigor. 



2. The mating of closely related individuals within a nor- 

 mally intercrossing population such as a breed of domesti- 

 cated animals, or a human population, is apt to cause some 

 loss of vigor. So much of the vigor of the population as is 

 due to its crossed (or heterozygous) character, will tend 

 gradually to disappear, as homozygous conditions are ob- 

 tained in consequence of inbreeding. The greater the num- 

 ber of characters in which a population varies, the slower will 

 be the attainment of a fully homozygous state in consequence 

 of inbreeding. If sufficient vigor is retained after a fully 

 homozygous state has been reached, then the closest inbreed- 

 ing (or even self-fertilization, when this is possible) should 

 cause no further loss of vigor. There is no reason to think 

 that monstrosities are produced by inbreeding (as for ex- 

 ample deformities, feeble-mindedness, insanity) except in 

 so far as such maladies may be due (1) to the lack of suffi- 

 cient vigor on the part of the organism to complete its normal 

 development, or (2) to the appearance in a homozygous state 

 of a recessive condition unseen in the heterozygous parents. 



3. The mating of individuals belonging to distinct geo- 

 graphical races of the same species of animal or plant usually 

 produces offspring larger or more vigorous than either parent 

 and fully fertile. The same result follows when distinct breeds 

 of domesticated animals or distinct varieties of cultivated 

 plants are crossed. The offspring are equal to or superior to 

 the parents in vigor and not less uniform in character. 

 But the F2 generation from such a cross does not retain the 

 superiority of the Fi generation, for it shows great variabil- 



