128 THE SHAPES AND SIZES OF ANIMALS 



size to be taken in as food, we can well imagine 

 how these might be passed on by the inner cells 

 into the central cavity, which will then become a 

 digestive cavity; the inner cells pouring out into 

 this cavity the secretions which dissolve the food, 

 and which it is their special purpose to manufacture. 

 The formation of a mouth by thinning away of the 

 wall at one point, will be a manifest advantage, as 

 it will avoid the necessity of the food particles 

 having to pass through both layers of cells in order 

 to reach the digestive cavity ; and on its appear- 

 ance the gastrula is completed. 



The Theory of Natural Selection requires that 

 each stage in the gradual evolution of a complex 

 organ or system should be a distinct, if slight, 

 advance on the stage immediately preceding it ; an 

 advance so distinct as to confer on its possessor an 

 appreciable advantage in the struggle for existence. 

 This condition is often overlooked ; we are apt to 

 assume, though most erroneously, that if it can be 

 shown that the ultimate stage is more advantageous 

 than the initial or earlier condition, then the 

 whole problem of the evolution of the organ in 

 question is solved. It is indeed seldom that we 

 are able to refer to so complete a series of inter- 

 mediate stages as that given above in the case of 

 the delaminate gastrula, each step being but a very 

 slight advance beyond the previous condition, and 

 yet each step conferring on its possessor a distinct 

 and tangible advantage. The fact that such a series 

 of forms can be pointed out, every one of which is 

 repeated in the life-history of certain jelly-fish, is a 



