174 DEVELOPMENT AND HEREDITY. 



to the forces of the environment only in one man- 

 ner. If it were torn into two pieces or into a dozen, 

 each could still react in only one given manner, and in 

 all cases with the same result. While there is but a 

 single plan of development possible to the larger 

 mass, yet each separate part of the mass has the 

 power of carrying out that plan. The potentiality 

 of development is divisible, and yet in each part 

 the potentiality is complete. The single plan of 

 development might, of course, be modified in some 

 of the separate pieces by exposing them to peculiar 

 conditions of environment. 



These inherited co-ordinations of growth, or the 

 inherited impulse of growth, as I have called it, 

 tend to guide the development of each separate 

 piece of a living mass in the same direction. The 

 living matter in each particular environment can 

 respond to the combination of forces acting upon 

 it in only one way. It repeats this response over 

 and over again, and thus has learned in its past 

 history only one plan, and it must always follow that 

 as nearly as the circumstances of environment will 

 allow. To this singleness of plan must we attribute 

 all such phenomena as the appearance of rudimen- 

 tary mammae in the male animal. These differ from 

 many other rudimentary organs in the fact that in 

 one sex they are necessary for the life of the species. 



