GROWTH AND REPRODUCTION 779 



circumstances to which their race has been for a long time exposed, 

 including the results of excessive use or disuse of an organ." 



Of recent years, the action of legislators, sociologists, and others, 

 has been directed to the belief that, by giving a good supply of fresh 

 air, exercise, proper sanitation, better education, the individuals will 

 grow up stronger and healthier, and thus provide a better race. But 

 will such methods convert bad stock into good stock ? The test lies 

 in the offspring. 



According to one school of thought, such environmental conditions, 

 although making improvement in the individual, will not better the 

 race. The hereditary factor is all-important. Such is the view of 

 Mendelism. 



The essence of the Mendelian principle is very easily expressed. 

 It is, first, that in a great measure the characteristics of organisms 

 are due to the presence of distinct, detachable character separately 

 transmitted in heredity; and, secondly, that the parent cannot pass 

 on to offspring a character which it does not itself possess. Each germ 

 cell, ovum, or sperm may contain or be devoid of any of these characters; 

 and since all ordinary animals and plants arise by the union of two 

 germ cells in fertilization, each resulting individual may obviously 

 receive in fertilization similar characters from both parents or from 

 neither. In such cases the offspring is " pure " bred for the presence 

 of the character in question, or for its absence. On the other hand 

 it may be developed from the union of dissimilar germs, one con- 

 taining a character, the other devoid of it; the individual is then 

 cross-bred, or heterozygous. A population thus consists of three 

 classes of individuals those pure-bred for the presence, having 

 received two doses, of a character; those pure-bred for the absence 

 of the character, having received none of it; and the cross-breds, 

 which have received one dose only. A plant, though cross-bred for 

 tallness, may be as tall as one pure-bred for tallness. A dwarf plant, 

 whatever be its parentage, can only produce dwarf offspring. Not 

 having tallness, it cannot transmit that property. A cross-bred tall 

 plant can, by self-fertilization, produce both tall and dwarf offspring. 

 Fowls with silky feathers cannot, if bred together, have offspring 

 with normal feathers, but two birds, normal to all appearance, 

 can, if they be cross-bred in that respect, produce silky off- 

 spring. 



These results are explained by assuming that a character may be 

 either dominant or recessive. In breeding, the transmission of these 

 characters is said to follow a definite law Mendel's law. When a 

 dominant and a recessive character are crossed, the first cross-bred 

 generation possesses the dominant character, which may be represented 

 as D(R) e.g., the cross between a tall and dwarf pea possesses the 

 dominant character of tallness. The issue of such cross-breds (impure 

 dominants) in the second generation will be 25 per cent, pure 

 dominant, 50 per cent, mixed (impure dominants), and 25 per cent, 

 recessive. In such a generation interbreeding of the dominants will 

 breed only dominants, of the recessives only recessives, but inter- 



