INTRODUCTION 



ii 



growth which causes us to grow into the individuals we are and 

 prevents us from being a mere mass of 160 pounds or so of 

 quivering jelly, evidently follows a definite rule with each kind 

 of living thing. The frog's egg always grows into a frog and 

 not into a dog or chicken. Our noses always grow on the fronts 

 of our faces and not between our shoulder blades. What causes 

 differentiation? any kind of differentiation which saves us from 



Figure 7. Nerve and muscle cells of animals. A, B, C, D, nerve cells. E, F, G, muscle cells. 

 After Sharp. By permission of McGraw-Hill Book Co. These are mature differentiated cells. 



the fate mentioned above? and what (in the second place) 

 causes it to follow a definite rule? Why do we always grow two 

 arms and not six or seven? and why do we grow hair on the tops 

 of our heads instead of the soles of our feet? These questions 

 cannot be completely answered. One answer is that there is a 

 vital force or principle which guides and shapes the clay of 

 which we are made. This force is supernatural. It is "some all- 

 controlling, unknown and unknowable, mystical, hyper-me- 

 chanical force." We cannot study it because it is supernatural, 

 beyond the limits of inquiry by natural means to which we as 

 natural and not supernatural beings are limited. The common 



