THE ARGUMENTS FROM EMBRYOLOGY. 459 



out with a greater surplus of vitality, and will be more likely 

 than others to live and propagate. Again, in the embryos of 

 its descendants, inheriting the tendency to economical trans- 

 formation, those which evolve at the least cost will thrive 

 more than the rest and be more likely to have posterity. 

 Thus will result a continual shortening of the processes. We 

 can see alike that this must take place and that it does take 

 place. If the whole series of phylogenetic changes had to be 

 repeated if the embryo mammal had to become a complete 

 fish, and then a complete amphibian, and then a complete 

 reptile, there would be an immense amount of superfluous 

 building up and pulling down, entailing great waste of time 

 and of materials. Evidently these abridgments which econo- 

 my entails, necessitate that unfolding embryos bear but rude 

 resemblances to lower types ancestrally passed through 

 vaguely represent their dominant traits only. 



From this principle of economy arise several derivative 

 principles, which may be best dealt with separately. 



130&. In some cases the substitution of an abridged for 

 an unabridged course of evolution causes the entire disappear- 

 ance of certain intermediate forms. Structural arrangements 

 once passed through during the unfolding are dropped out of 

 the series. 



In the evolution of these embryos with which there is not 

 laid up a large amount of food-yolk there occurs at the outset 

 a striking omission of this kind. When, by successive 

 fissions, the fertilized cell has given rise to a cluster of cells 

 constituting a hollow sphere, known as a Uastula, the next 

 change under its original form is the introversion of one side, 

 so as to produce two layers in place of one. An idea of the 

 change may be obtained by taking an india-rubber ball (hav- 

 ing a hole through which the air may escape) and thrusting in 

 one side until its anterior surface touches the interior surface 

 of the other side. If the cup-shaped structure resulting be 

 supposed to have its wide opening gradually narrowed, until it 



