DOMESTIC ANIMALS 



37 



effect of environment upon the animal di- 

 minishes rapidly from the time of its 

 conception. The condition of the dam 

 at the time of impregnation is of great 

 importance, as is also her state of health 

 and nutrition during the period of preg- 

 nancy. 



Domestication and variation — The 

 amount of variation in growing ani- 

 mals is increased as the complexity of 

 the environment increases. Domesti- 

 cated animals vary more rapidly than 

 wild species for the reason that the feed 

 is more varied, other external conditions 

 change more frequently, many new con- 

 ditions arise and variations in the animal 

 are quickly noted and utilized by 

 the breeder. Domestication has brought 

 about important changes in animals. 



domestication animals apparently be- 

 come more plastic and subject to varia- 

 tion. But the difference in this respect 

 is not great and is perhaps partly due to 

 the fact that we can readily note the 

 changes in domesticated animals on ac- 

 count of having them under constant 

 observation. 



Temperature exercises a direct effect 

 on the growth, size and temperament of 

 animals as well as upon the nature of 

 their coat of hair. For most animals 

 there is an optimum temperature at 

 which they thrive best and a minimum 

 and maximum beyond which they cannot 

 live. The change to a white coat in 

 winter noted in ptarmigan, hares, etc., is 

 the direct effect of cold. The change 

 takes place first on those parts of the 



Fig. 24 CHESHIRE SOW OF MODERN TYPE 



(Sweepstakes at Pan American Exposition. Age six years, weight 594 pounds.) 



In some instances the digestive tract 

 has been greatly enlarged and the food 

 consuming capacity correspondingly in- 

 creased. Moreover, the fecundity has 

 been increased and the breeding period 

 has become extended over a much longer 

 season than in the wild relatives. In 

 some animals all periodicity in breeding 

 has disappeared and they will breed at 

 any time. Again the polygamous rela- 

 tion is much more common in domesti- 

 cated than in wild animals. Rabbits in- 

 crease in size under domestication, the 

 weight of the wing and leg bones of 

 fowls decreases. These changes make 

 the animals in question better suited to 

 man's purposes and are direct adapta- 

 tions to the domesticated state. Under 



body in which the blood circulation is 

 least active. 



One of the effects of intense light is 

 the development of more pigment in the 

 skin. In the tropics the skin even of 

 the white races becomes brown. In the 

 southern states white hogs suffer from 

 sun blisters. Black or red breeds are 

 not thus affected. 



Influence of feed — The quality and 

 amount of feed is one of the most 

 important factors in the variation of 

 animals. Many instances of the in- 

 fluence of food have been collected 

 by Vernon. Hemp seed gives some 

 birds a black plumage. Cayenne pep- 

 per in the food changes the color of 

 the feathers from yellow to orange red, 



