460 THE EVOLUTION OP LIFE. 



becomes the mouth of an internal chamber, it will represent 

 what is known as a gastrula a double layer of cells, of which 

 the outer is called epiblast and the inner hypoblast (answering 

 to ectoderm and endoderm) inclosing a cavity known as the 

 archenteron, or primitive digestive sac. But now in place of 

 this original mode of forming the gastrula, there occurs a 

 mode known as delamination. Throughout its whole extent 

 the single layer splits so as to become a double layer one 

 sphere of cells inclosing the other; and after this direct for- 

 mation of the double layer there is a direct formation of 

 an opening through it into the internal cavity. There is thus 

 a shortening of the primitive process: a number of changes 

 are left out. 



Often a kindred passing over of stages at later periods 

 of development may be observed. In certain of the Mollusca, 

 as the Patella chiton, the egg gives origin to a free-swimming 

 larva known as a trochosphere, from which presently comes 

 the ordinary molluscous organization. In the highest division 

 of the Molluscs, however, the Cephalopods, no trochosphere is 

 formed. The nutritive matter laid up in the egg is used 

 in building up the young animal without any indication of an 

 ancestral larva. 



130c. Among principles derived from the principle of 

 economy is the principle of pre-adaptation a name which we 

 may appropriately coin to indicate an adaptation made in 

 advance of the time at which it could have arisen in the 

 course of phylogenetic history. 



How pre-adaptation may result from economy will be 

 shown by an illustration which human methods of con- 

 struction furnish. Let us assume that building houses of 

 a certain type has become an established habit, and that, 

 as a part of each house, there is a staircase of given size. And 

 suppose that in consequence of changed conditions say the 

 walling in of the town, limiting the internal space and in- 

 creasing ground-rents it becomes the policy to build houses 



