324 Dr. T. Margé on the Classification 
as we have no indubitable grounds or facts which directly 
indicate the retrogression or degeneration of a given animal- 
form or group, we shall proceed most securely in judging of 
its derivation if we for the time assume it to have been pro- 
gressive. 
Nevertheless there are animals with regard to which we 
may assume, if not with certainty at least with much proba- 
bility, that they have originated by retrograde development 
from other more perfect animals. Thus, among others, it 
seems very probable that the Dicyemide, which EK. van 
Beneden regards as an independent progressive animal-form 
standing between Protozoa and Metazoa, and refers to the 
group Mesozoa, which he has established, are really only 
forms of some Platyelminth, degenerate in consequence of a 
parasitic mode of life; just as the Myzostoma are Chetopod 
worms degenerated by retrograde metamorphosis. It is, more- 
over, probable, although not certain, that among the Protozoa 
forms occur which possibly owe their existence to a retrograde 
metamorphosis and have originated from Metazoa. Nay, the 
possibility does not seem to be excluded that even the Polyps 
and Corals belong to the retrogressive animal-forms, and per- 
haps are nothing else than the peculiarly modified and dege- 
nerated descendants of some originally free-swimming bilateral 
form of worm. Even in the great group of the Vertebrata 
it is a highly interesting question whether Amphioxus and 
the Cyclostomt are not the descendants of some more highly 
organized craniate Protovertebrate, constructed after the type 
of the Monorhina, which has become in course of time entorely 
extinct, and in which both the jaws and the paired limbs were 
still wanting. 
3. Besides the difficulties here mentioned which come in 
our way in the classification of the animal kingdon, but which 
seem to be superable sooner or later by further investigation 
and unwearied labour, we must consider one obstacle, the 
power of which the human mind will never be able completely 
to overcome. 
Thus, for a truly genealogical and phylogenetic classification 
of the animal kingdom we need not only an accurate knowledge 
of the existing forms, but also of those which have long since 
been extinct, and, further, a comparison of these forms with 
each other, for paleontological facts, as is well known, are of 
no less value than the knowledge of the embryological and 
morphological characters of living animals. 
According to Darwin’s developmental law, we may con- 
clude that very numerous ¢éransitional forms between the 
different groups of animals must formerly have existed; this 
