Zoology. 123° 
tion has been paid. 
When comparing, in former years, the characters of fossil fishes, 
especially with a view of ascertaining their natural relations to the living 
types, | was struck with the fact that those of earlier ages presented 
Many structural peculiarities, which occur only in the embryonic con- 
dition of the fishes of our days, and also that the older representatives 
of any family rank lower in comparison to their living representatives. 
us led me to infer that embryonic data might be applied with ad- 
Vantage 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. ae 
nder 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 | 
in their full grown condition. 
This principle, once ascertained, led to the result, upon more exten- 
Sive investigations, that a complete knowledge of the metamorphoses 
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. tes ae 
Beginning the revision of the animal kingdom with the type of the 
piticulata, it was not difficult, with these views, to ascertain that the 
oO rg d pa Z 
