Classification in Zoology. 229 



"When comparing, in former years, the characters of fossil 

 fishes, especially with a view of ascertaining their natural 

 relations to the living types, I was struck with the fact, that 

 those of earlier ages presented many structural peculiarities, 

 which occur only in the embryonic conditions of the fishes of 

 our days, and also that the older representatives of any family 

 rank lower in comparison to their living representatives. 



This led me to infer that embryonic data might be applied 

 with advantage to the correct appreciation of the natural re- 

 lation of the various members of one and the same family, 

 and perhaps also to the determination of the relative posi- 

 tion of closely allied types. 



Under this impression, I began to compare young animals 

 of various families with the different types of the same family 

 in their full-grown condition, when I was forcibly struck with 

 the close resemblance there is between the younger stages of 

 development of such representatives as could otherwise be 

 recognised 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 i 

 standard to determine the natural order of succession of all 

 the representatives 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 extensive investigations, that a complete knowledge of 

 the metamorphoses of animals, from the earliest period of 

 their embryonic development to the last change they under- 

 go 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 the 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 struc- 



