112 "THE BIOLOGICAL PROBLEM OF TO-DAY 



m 



ence to their different requirements, increase takes 

 place in the internal surface. The specialisation of 

 plants displays itself in organs externally visible in 

 leaves, twigs, flowers, and tendrils. The specialisa- 

 tion of animals is concealed within the body, for the 

 internal surface is the starting-point for the forma- 

 tion of the organs and tissues. 



Comparative embryology shows that, however 

 varied the forms and functions of the numerous 

 animal organs may be, the method of their develop- 

 ment is remarkably similar. There are required 

 only the slightest variations of a few simple general 

 laws. For these I may refer readers to a series of 

 special investigations (Studies on the tierm-layer 

 Theory, Oscar and Kichard Hertwig), and to the 

 fourth chapter of my Embryology, ' General Dis- 

 cussion of the Principles of Development.' 



In these works and in the foregoing pages I have 

 tried to show that the multiplication of the egg- 

 cell by division is itself a source of increasing com- 

 plexity and an active principle in the determination 

 of form, since the products of the division unite to 

 form a higher unity. But in another way the 

 multiplication of cells leads to differentiation among 

 the cells arising from the egg. Although each of 

 these resembles the parent egg, from which they 

 arose by doubling division, yet they differ from it 

 in one point : they are no longer a whole, but have 

 become the subordinate parts of a higher unity, 

 that is, of a higher organism. A cell that is no 

 longer a whole, but the part of a whole, has entered 

 upon reciprocal relations with other cells, and in 



