154 EVOLUTION' OF TO-DAY. 



Embryological History. 



Having now noticed these various difficulties in 

 the way of a complete history, let us begin with 

 early stages of development and see how far any 

 logical history can be traced. Of the early stages 

 of development embryologists have made us well 

 acquainted, for they have spent most of their time 

 on this subject. Owing to the difficulties of observa- 

 tion upon older embryos, it is the early stages which 

 have received most attention. But this necessity 

 has not been an unfortunate one. It is evident, 

 from the fact that all of the great groups of animals 

 were already developed in the earliest rocks, that 

 paleontology can tell us nothing in regard to the 

 development of these groups, and their relation to 

 each other ; a fact which has been used as an argu- 

 ment for the primitive distinction of types. Now, 

 embryology comes in to fill up this lost history. 

 The early stages of the embryo must, of course, 

 represent the first stages of this lost record, and 

 ought to give the relation of the sub-kingdoms to 

 each other. The later stages represent later history 

 and the origin of smaller groups. The latest stages 

 will represent the growth of the smaller groups of 

 genera. Now, the study of classification gives good 

 evidence as to the origin of small groups; paleon- 

 tology enables us to trace the history of the origin 

 of orders, etc. ; but embryology alone can give us 

 evidence as to the relation of the sub-kingdoms to 

 each other. It is, then, a fortunate necessity which 

 has led to a particularly careful study of early stages 

 of the embryos of a very large number of animals; 



