* 
Dana on the position of Amphibians among Vertebrates. 185 
on Classification,’ criticizes the systems of Van Beneden, Kolliker 
and Vogt, on account of their violating the structural affinities 
of groups, implying that embryological conclusions have to be 
tested by a reference to the natural types of structure. In nature 
a specific type is often expressed in a long series of species run- 
ning through a very wide range of grade; and structures so di- 
verse in grade as those of the higher and lower extreme groups 
are diverse in the nature of the changes which take place in the 
course of embryological development. Not appreciating this 
fact, enbryological systemists have cut the series, and mad¢ bol 
demarcations between parts that are essentially one in type. 
Thus has resulted the separation of the class of Worms from 
Articulates by both Van Beneden and Vogt, and of the order 
of Cephalopods from Mollusks by the latter, ete.; and such 
errors will continue to attend upon the decisions of pure embry- 
ology until the precise value of its characteristics in classification 
Is understood. : 
If, then, the structural relations of the developed animals are 
an authority to which embryology must appeal, the adult Am- 
phibians may claim to be considered, on a question of their rela- 
tions to ordinary Reptiles, even before their eggs and young. 
mbryology proves that Amphibians and ordinary Reptiles are 
distinct groups, as is proved also by structural considerations; 
ut, in the present state of the science, it can hardly be said to 
demonstrate that these groups are classes, coUrdinate with those 
of Birds and Mammals;—and I venture to say, as regards the 
gl of groups, that, in no state, will it prove what the 
adult structures will not sustain. 
nary or terrestrial Reptiles? The development, at each step, in- 
Volves, and depends upon, chemical changes; and it is hence 
Cetaceans. pet 
‘divides the Invertebrates into two groups, the first, including Insects, Myria- 
- agi and Crustaceans, the second, the eabkingdom = ceorce aye the inferior 
mo the subkingdom of Articulates, that is, Worms, toge he ltadiates, 
Rhizopods and mea ‘and his division of Polyps, among the Radiates, in his 
i s both Polyps and Acalephs. Vogt makes 
: in Tading Vertebrates, and all Articulates 
excepting Worms ; the second, Mollusks, Worms and Radiates; the thi — 
it : ; and his division of Mollusks does not embrace the Cep. w we 
it does include a tribe of Acalephs. Recently, Prof, Huxley, in lectures before 
Gaze’ College of S s, of which a report is given in the Medical and 
tte, for May, 1863, says, (page 555,) after discussing | the im 
Placenta in Mammals as a basis of classification, that, in his view, t 
culty in the way of a classification which unites the Pro ideans with 
tather than with Paridigitate and Imparidigitate Herbivores. 
there is no diffi- 
the Rodents 
