5 



This led me to infer that embryonic data might be applied with 

 advantage to the correct appreciation of the natural relations of the 

 various members of one and the same family, and perhaps also to the 

 determination of the relative position of closely allied types. 



Under this impression, I began to compare young animals of vari- 

 ous families with the different types of the same family in their full 

 grown condition, when I was forcibly struck with the close resem- 

 blance there is between the younger stages of development of such 

 representatives as could otherwise be recognized as ranking high in 

 their respective families, and the lower forms belonging to the same 

 groups. This led naturally to the conclusion, that the change which 

 animals undergo during their growth might safely be taken as a stan- 

 dard to determine the natural order of succession of all the repre- 

 sentatives of any given type, within the limits in which the higher 

 ones pass successively through transient forms, which the lower ones 

 naturally present permanently in their full grown condition. 



This principle, once ascertained, led to the result, upon more ex- 

 tensive investigations, that a complete knowledge of the metamor- 

 phoses of animals, from the earliest period of their embryonic 

 development to the last change they undergo before reaching their 

 mature condition, would afford, throughout the animal kingdom, a 

 true measure by which to ascertain precisely, and without arbitrary 

 decision on our own part, the natural relative position of all the 

 minor groups of the animal kingdom. , 



Beginning the revision of the animal kingdom with the type of 

 Articulata, it was not difficult, with these views, to ascertain that 

 the Worms, as a natural type, rank lowest in this department, as 

 they represent permanently a structural adaptation which is closely 

 analogous to the earliest condition of the development of the Insects ; 

 that the Crustacea constitute a class intermediate between the Worms 

 and Insects, and not superior to the Insects, as some naturalists 

 would have them ; inasmuch as the highest combination of their 

 rings presents us with an arrangement similar to that of the pupa of 

 Insects, in which the joints of the head and of the chest are combined 

 in an immovable shield, as in the pupa of Insects, and in which the 

 joints of the abdomen alone remain movable, is also the case among 

 the highest Crustacea. The position of the Insects as the highest 

 class can no longer be denied, when we consider that in them the 

 body is at last divided into three distinct regions head, chest and 

 abdomen and that the locomotive appendages, which, in the lower 



