of the Animal Kingdom. 321 
animal forms, as well as those which live under ground, in 
mud, in dark caverns, and at great depths in the sea, or which, 
firmly adhering to other bodies, are incapable of any locomo- 
tion. We may refer here only to the Trematoda and Cestoda 
among the Platyelmintha, which evidently are nothing but 
the direct descendants of Turbellaria once leading a free 
existence, but gradually retrograded in consequence of a para- 
sitic mode of life. The Cirripedia and Rhizocephala among 
Crustaceans, and the Linguatulides among the Arachnida, are 
nowadays justly regarded as such animal forms produced by 
retrograde metamorphosis, as alsothe Bryozoa, Brachiopoda, &e. 
It may, however, not always be easy to recognize with 
certainty the cases of retrograde phylogenesis, or degeneration, 
and probably many forms which at present are ascribed to 
retrograde metamorphosis may in course of time prove to be 
progressive animal forms. 
As in all other things, so also in science, man is inclined to 
overstep the bounds of sober reason, and to assume or assert 
& priort more than what facts directly prove. This applies 
so much to the theory of retrogression or degeneration, so widely 
diffused at the present day, that a natural philosopher has even 
ventured to set up the proposition that all animal forms are 
direct descendants of man, gradually degenerated by retro- 
gressive metamorphosis ! 
Another view, which I believe to be erroneous, is also 
entertained by some savants, namely, the opinion that the 
formation of body-segments (metameres), or the repetition of 
similar parts of the body, always leads towards the perfection 
of the organism. In accordance with this assumption many 
naturalists are inclined to derive the unsegmented animal- 
forms, or those destitute of metameres, from supposititiously 
more perfect and allied animals, in which the body was once 
composed of numerous segments or metameres, and therefore 
originally segmented. Prof. Semper, of Wiirzburg, as is well 
known, has lately promulgated a similar opinion*,. namely, 
that the Mollusca are to be regarded as the direct descendants 
of the Annelida, and consequently that the unsegmented 
Molluscan form has gradually been developed by retrograde 
metamorphosis from the Annelidan form. While it is pos- 
sible, or even probable, that in many cases the formation of 
metameres, or the frequent repetition of equivalent parts, may 
lead towards perfection, observation often seems to prove the 
contrary. Thus, for example, among the Mollusca, we find 
generally in the Cephalopoda and Gasteropoda not the smallest 
trace of metamere formation (with the exception of the ex- 
* Op. cit, supra, p. 315, note. 
