256 THE BREEDING OF ANIMALS 



It is not too much to say that no man can be a successful 

 breeder who is not also skillful in developing his animals. 

 Thus, in practice, development becomes supremely im- 

 portant and throughout the history of animal-breeding 

 has been only second in importance to heredity itself. 

 A satisfactory treatment of development in all its phases 

 as related to animal husbandry would require a volume, 

 and as the chief purpose of this work is to consider how 

 the inherited capacities of animals fin ally appear as definite 

 characters in the germ-plasm, only a limited reference 

 can be made to developmental phases of chief importance 

 to the animal-breeder. 



241. Growth. From the fertilization of the egg until 

 the full development of the mature individual, the animal 

 increases in volume and changes in form. This increase 

 and change of form is called growth. The final size of 

 an animal is determined by the rate of growth and the 

 length of the growth period. The guinea pig and rabbit 

 come to full maturity at about the same age, but the rabbit 

 is larger because its rate of growth is more rapid. The 

 rate of growth in the rabbit and man is about equal, 

 but man is much larger at maturity because the period 

 of growth is much longer. 1 



242. The growth impulse. The young of any species 

 tend to develop and grow in accordance with the normal 

 habit of the species. This applies in a special sense to 

 the skeletal system. Even in the absence of a sufficient 

 supply of feed and other favorable conditions, the young 

 animal displays a remarkable physiological impulse to 

 continue to increase in the skeletal parts. 2 Animals fed 



1 Morgan, "Experimental Zoology," p. 245. 



2 Waters, "Capacity of Animals to Grow under Adverse 

 Conditions." 



