126 DEVELOPMENT AND HEREDITY. 



such an organism as the Hydra the effects of irrita- 

 tion spread in circles from the point stimulated, 

 whereas in the mammal they are carried in definite 

 channels to the nervous centre, and from there 

 transmit their effects throughout the body, the 

 nervous centre acting like the banker's clearing- 

 house in adjusting the balance of credit. Thus we 

 see that the physiological union between the parts 

 and organs of a mammal is as complete as the 

 union between the parts of a unicellular organism. 

 There is no reason to suppose that a change can 

 be produced in one part of a multicellular organism 

 without affecting in some way the entire organism. 

 But there remains a difference between the uni- 

 cellular and multicellular organisms in regard to 

 reproduction and inheritance. In the first the 

 organism simply divides into a number of pieces 

 of protoplasm, and each piece possesses the char- 

 acter of the entire mass. In the second, particles 

 of protoplasm are separated off from the organism 

 and continue to live and grow into new individuals, 

 while the main body of the parent organism remains 

 unchanged for a period, and then dies. An under- 

 standing of this phenomena is best obtained by 

 following the generally accepted view that the 

 metazoa are descended from the protozoa. The 

 course of this evolution must have been something 



