THE ORIGIN OF THE ANIMAL KINGDOM. 75 



the same as those supposed to have been in the 

 early primitive animal. Among other coelenterates 

 this simple type is more or less modified, but it can 

 always be seen to have the same fundamental plan. 

 Since, then, the lowest existing group of multicellu- 

 lar animals is essentially the same in structure as the 

 Gastraea, to which embryology teaches us to refer all 

 animals, we must conclude that the former existence 

 of the Gastraea, as the starting-point of multicellular 

 animals, is one of the best attested facts of biological 

 science. It is a conclusion that can be gainsaid only 

 by denying completely the value of embryological 

 and anatomical evidence, and this would of course 

 be to deny the cogency of biological science com- 

 pletely. 



The first step in the history of life toward the 

 development of the higher animals may then be 

 briefly summarized as follows : Some original uni- 

 cellular animals, in the course of their repeated 

 divisions, failed to separate completely into single 

 cells after dividing, but the cells thus produced 

 remained attached to each other. At first the cells 

 were all alike, and had similar functions, but after a 

 time the cells on the outside and those on the inside 

 of the mass began to perform different duties. 

 Those on the inside could no longer use any powers 

 of motion, and the possession of sensitive functions 

 would be useless, since they had no direct relations 

 with an external world to excite the sensations. It 

 is a law of nature that any power which is not used 

 begins to degenerate, and therefore the internal 

 cells soon lost their motor and sensory functions. 



