1294 THE STORY OF THE UNIVERSE 



ferent, as respects its inner and outer envelope. 

 But these envelopes are at present uniform; one 

 part of each is exactly like the rest Let us, there- 

 fore, follow the history of Development, and we 

 shall find that the inner wall gradually becomes un- 

 like itself in various parts, and that certain organs, 

 constituting a very complex apparatus of Digestion, 

 Secretion, and Excretion, are all one by one wrought 

 out of it by a series of metamorphoses or differentia- 

 tions. The inner wall thus passes from a simple 

 assimilating surface to a complex apparatus serving 

 the functions of vegetative life. 



Now glance at the outer wall: from it also vari- 

 ous organs have gradually been wrought; it has de- 

 veloped into muscles, nerves, bones, organs of sense, 

 and brain all these from a simple homogeneous 

 membrane! 



With this bird's-eye view of the course of develop- 

 ment you will be able to appreciate the grand law 

 first clearly enunciated by Goethe and Von Baer as 

 the law of animal life, namely, that development 

 is always from the general to the special, from the 

 simple to the complex, from the homogeneous to the 

 heterogeneous, and this by a gradual series of differ- 

 entiations. 



Here is our Opalina, for example, without mouth, 

 or stomach, or any other organ. It is an assimilating 

 surface in every part; in every part a breathing, 

 sensitive surface. Living on liquid food, it does not 

 need a mouth to seize or a stomach to digest such 

 food. The liquid, or gas, passes through the Opa- 

 lina's delicate skin by a process which is called en- 



