108 THE BIOLOGICAL PROBLEM OF TO-DAY 



would understand the causes by which growth 

 beyond a definite limit must result in invagination. 

 From the occurrence of the gastrula in all the 

 divisions of the animal kingdom, we may conclude 

 that it is a temporary phase, inevitable in the 

 growth of animals. 



There may be noticed here a second connection 

 between shape and organic growth, exceedingly 

 simple in its nature, but of fundamental importance 

 in its consequences. It may be stated in this saying : 

 Growth always must be such as to produce the 

 greatest possible extension of surface. The reason 

 of this is simple, depending on the different natures 

 of inorganic material and living organic material. 



A crystal in its mother liquor grows by attract- 

 ing new particles and depositing them upon its 

 outer surface, according to the kind of crystallisa- 

 tion peculiar to the material of which it is com- 

 posed. These particles, once crystallised, retain 

 their position even when new layers are deposited 

 on their outer surfaces, and remain unchanged, 

 perhaps, like rock crystals, for thousands of years, 

 until changed outer forces loosen the bonds that 

 bind them. 



Organised material cannot grow in this fashion ; 

 it takes up material from without, not, like the 

 crystal, arranging it on the outer surface, but in- 

 gesting it. Protoplasm cannot become fixed in any 

 condition without being destroyed ; it exhibits 

 perpetual interchanges with the outer world ; un- 

 ceasing intake and output is a necessary accompani- 

 ment of its life. ' The growth of idioplasm,' as 



