320 Dr. T. Margé on the Classification 
plasy or adaptive homology we also find the explanation of 
the formation of segments which occurs in many Platyel- 
mintha, which therefore, as a simple phenomenon of adaptation, 
ean by no means be equivalent to the metamere-formation of 
the Annelida. As a homoplasy of the same kind we also 
regard the resemblance between Rotatoria and Crustacea, 
which is erroneously interpreted by some authors, as well as 
that between the bills of birds and the maxillary horny 
sheaths of the Chelonia, and many others. Many erroneous 
views in this respect have already been refuted by more 
accurate investigations. 
2. In consequence of Darwin’s researches it is generally 
recognized as the law of development in the organic world, 
that all organisms, animals as well as plants, are in a state 
of constant, although very slow, change and progressive deve- 
lopment, so that, in general, the effort to attain greater perfec- 
tion ts predominant in the organic world. Fyrom this con- 
stantly advancing progressive movement of phylogenesis 
many have erroneously concluded that all the simpler organ- 
isms living at the present day are only the representatives of 
similar animals which have continued in existence from an 
earlier period, and that consequently those natural groups to 
which they belong have never been at a higher stage of 
organization than that upon which they now stand. 
In accordance with this principle many were at first of 
opinion that the whole animal kingdom, as well as its larger 
and smaller groups, might be quite simply embraced in several 
ascending series, just as these, in consequence of constant 
and unceasing advance towards perfection, had gradually 
become developed from the lowest to the highest stage of 
animal organization. It was soon found to be impossible, 
however, to establish the true genealogical tree of the animal 
kingdom upon this principle without falling into faults and 
errors. For Darwin’s law of development, which in its final 
results is certainly progressive, by no means eacludes a retro- 
grade movement or retrogression in certain stages of develop- 
ment, as life presents not only progressive but also retrograde 
phenomena of movement, and absolute rest or unchangeability 
(Stabelismus) is an impossibility in Nature as in social life. 
In accordance with this law, by the action of unfavourable 
conditions of existence, and by means of natural selection, 
perfectly organized higher organisms may originate much 
simpler forms, which are, so to speak, degenerated by retro- 
gression, although for this very reason better fitted for and 
corresponding to the new conditions of existence. 
Of late the parasitic animals have been regarded as such 
