of the Animal Kingdom. 315 
as is well known, there are Natural-history systems, such, for 
example, as the systems of the old school, which were chiefly 
founded upon the external form and vital relations of the 
animals. There are systems chiefly founded upon anatomical 
facts, such as the system proposed at the beginning of the 
present century by Cuvier, who divided the whole animal 
world into four principal types (Vertebrata, Mollusca, Articu- 
lata, Radiata) ; and there are also systems established upon 
embryological facts, such as that of C. Semper, who, as is well 
known, has quite recently attempted to determine accurately 
the relationships of animals according to the embryonal prim1- 
tive kidneys or segmental organs, and to classify the Metazoa 
phylogenetically upon this exclusively embryological basis *. 
The discovery of segmental organs (primitive kidneys) in 
the embryos of sharks, for which we are indebted to Professor 
C. Semper, and in part also to the English embryologist F. 
Balfour, is doubtless so far of great importance to the trans- 
formation-theory, that it furnishes a fresh proof of the fact 
that the Vertebrata and Evertebrata, namely the Vermes, 
possess in common organs which are homologous from a 
morphological point of view. 
It is, however, questionable whether it is permissible to 
ascribe to the above-mentioned organs so great an importance, 
and to found upon them exclusively a classification of the 
entire animal kingdom. I would ask, is the relationship 
between the other Vertebrata and the Annelida really greater 
than between the other Vertebrata and Amphioaus, or between 
this latter and the Ascidia? 
Many zoologists believe that they can give science a new 
direction by selecting according to their judgment one of 
the embryonal organs, or any phenomenon of the embryonal 
organism, in order to found upon it a phylogenetic classifica- 
tion, in the hope of being able in this way to establish a 
natural relationship of the groups of animals. We may 
readily admit that embryonal characters are of greater value 
in taxonomy than characters of the fully developed animals 
derived from anatomy or biology, as in the latter the characters 
may much more easily suffer change by adaptation and change 
of function; but we think we may assert with certainty that 
embryonal characters taken by themselves (even if they relate 
to the most important organ of the embryo) are far from being 
sufficient for the sure establishment of the descent of one of 
the smaller groups of animals, and still more of the whole 
animal kingdom. 
* ©. Semper, ‘Die Verwandtschafts-Bezichungen der gegliederten 
+ Thiere’ (Hamburg, 1876), 2 vols. 918 
