296 WALKS AND TALKS. 



present time. The continent grew by continuity, and im- 

 proved by ever increasing specialization. The great delta 

 started at a point and grew by additions to itself, and by 

 ever widening relations to the land, and ever increasing com- 

 plications of structure. So every feature of the earth grew. 

 The whole history of the physical world exemplifies a method 

 of specialization tiirough continuity. 



Consider next, the realm of organic matter. Under what 

 method does Nature produce an animal or a plant? By 

 what method has the world been populated? What method 

 has been pursued in the geological succession of organic forms 

 which we have passed in review? 



To learn by what method Nature produces an animal, we 

 must trace its history from the earliest condition in which it 

 I exists. Every animal exists first as an egg that is, a certain 

 amount of yolk with an included cell in which is a point 

 where the force is seated which we will call "vital," and 

 * which, though inscrutrable, is the essential part of the egg. 

 In viviparous animals, the eggs are developed within the 

 parent. In any order of animals, when the conditions are 

 suitable, the yolk, whether of an external or an internal egg, 

 begins to undergo changes. The yolk, or a portion of it, 

 divides into two parts ; then each of these, into two ; and this 

 subdivision proceeds until the whole yolk, or the divided por- 

 tion of it, is a mass of globules aggregated together, and pre- 

 senting the appearance of a mulberry. On and within this 

 mass, a disc (blastoderm) appears, consisting of two layers, 

 the upper (epiblast) formed of nucleated cells, and the lower 

 (mesoblast) of irregularly rounded cells. From the upper are 

 destined to be developed the skin and walls of the body, 

 together with the nervous chord ; from the lower, the heart 

 and vascular system, the stomach and intestines. I will only 

 mention a few features in the development. The lower layer 

 undergoes progressive changes. We perceive the f:u 

 rudiments of head and tail, ami abdominal cavity. The out- 

 lines of the neural canal appear, and of some of the first ver- 

 tebrae. Now the lower layer splits, and one part proceeds to 



