THE ORIGIN OF THE ANIMAL KINGDOM. 8 1 



seed springs from the ground, really determines the 

 subsequent shape of the tree. No matter if the tree 

 lives to be a hundred years old it will always be pos- 

 sible to see in its giant limbs the early branching of 

 the seedling. These early branches become larger 

 ones, and they in turn give rise to smaller branches 

 and twigs, but after the first few weeks' growth it is 

 impossible for the plant to produce any more pri- 

 mary branches. So in a modified way it seems true 

 in the animal kingdom. Very early in the history of 

 animals, the Gastraea trunk branched into several 

 primary divisions, and these have continued to the 

 present time. They have grown larger, they have 

 produced many subdivisions, but the animal king- 

 dom did not, after the early periods of its history, 

 produce new primary divisions. In other words, no 

 new sub-kingdoms have arisen since the earliest 

 periods of the development. (For Vertebrata see 

 Chapter IV.) 



It is possible to reach this same conclusion from 

 another standpoint. The development of the ani- 

 mal kingdom has been in all cases from undifferen- 

 tiated to differentiated. Organs originally all alike 

 and adapted to simple functions, have become dif- 

 ferent from each other and adapted to more com- 

 plex functions. For instance, the mass of muscles 

 which in the fish's body are adapted only to the 

 flexion of the body from side to side, and the simple 

 motions of its fins, become in the more developed 

 vertebrates, like man, divided into several hundred 

 separate muscles, each with its own function, all result- 

 ing in the complicated powers of motion shown by 



