572 APPLIED BIOLOGY 



grow horns) would probably transmit the tendency towards 

 absence of horns to a considerable percentage of her off- 

 spring, and these, in turn, would tend to produce hornless 

 offspring. In fact, the breeds of hornless cattle, which are 

 now becoming popular among farmers, have been developed 

 by selecting for breeding certain individuals born without 

 the beginnings of horns. Likewise, dogs and cats born with 

 short tails are likely to transmit that germinal characteristic, 

 which in some unknown way is carried in the reproductive 

 cells from parent to offspring. 



The above paragraph states the facts verified by careful 

 observation in hundreds of cases. In fact, the numerous 

 varieties of domesticated animals and plants have originated 

 by man's selection of individuals which during their em- 

 bryonic history began to develop peculiarities. If these 

 peculiarities make the individual decidedly different from 

 its parents, a new breed might be originated, as from the 

 first hornless cow from horned ancestors. Usually, however, 

 the peculiarities are little things which, if selected by man, 

 will make an improvement in the breed. Hence the scientific 

 farmer is continually watching for young animals which show 

 some slight improvement over their parents; and he goes 

 through his fields in search of corn and other plants which 

 are better than the others. This, in brief, is the secret of the 

 remarkable improvement in almost all kinds of farm animals 

 and cultivated plants in the past fifty or one hundred years. 



502. Heredity and the Germ-cells. Since an offspring 

 resembles each of its parents (a fact especially striking when 

 the two parents belong to different varieties and the off- 

 spring is a hybrid) , it is evident that the egg-cell and 

 the sperm-cell which united to produce the new individual 

 were the bearers of heredity. In fact, it has been shown 

 that the chromosomes in the nuclei of the egg-cell and sperm- 

 cell contain the hereditary substance, the nature of which is 

 unknown. Since each germ-cell contributes one-half of the 



