8O THE LIVING WORLD. 



vertebrates are advanced. The relations of the star 

 fishes (Echinoderms)) the group of low worms 

 (Vcrmes), and some other types still prove a puzzle 

 to him, and it will doubtless require much investiga- 

 tion still before the embryological history is fully 

 elucidated. 



The history of the development of the primitive 

 Gastraea into our modern types is a matter of specu- 

 lative interest to the specialist, but to the general 

 reader is too complex to make it worth while to 

 dwell upon it. There is, however, concealed in this 

 subject a general principle of development of the 

 most extreme importance. From the history of this 

 Gastraea we learn that the divergence of the great 

 types of animals must have occurred early in the 

 history of animals, and that no new great types have 

 appeared except in the early history. The reason for 

 this is easily seen. The differences which separate 

 the great types of animals from each other are in 

 points of fundamental structure and plan. Such dif- 

 ferences could have appeared only in the descend- 

 ants of some early form in which no special type had 

 appeared. After the line of descendants had assumed 

 a definite type, it is not likely that they would ever 

 afterwards have changed their type though many 

 changes in minor details may have occurred. In- 

 deed, all evidence shows us that the types, after 

 they have once become fully established, have not 

 changed their plan but have remained constant. A 

 tree when it starts from the ground as a seedling 

 soon gives rise to several branches. Now this early 

 branching which takes place in a few days after the 



