of the Animal Kingdom. 323 
the above mentioned animals are surprisingly similar, and 
agree in most of their characters, but, above all, for the special 
and important reason that there seems to be evidence that the 
chorda which occurs in nearly all Ascidia in the larval period 
(with the sole exception of the Molgulide) can by no means 
be regarded as an organ acquired by adaptation during the 
larval period, but as one which they have obtained originally 
by inheritance from their formerly more perfectly organized 
ancestors. We may therefore, from the transitory presence 
of the chorda, and its, in most cases, complete retrogression at 
a later period, draw the correct conclusion that the Tunicata 
were not produced by progressive phylogenesis, but that they 
must have originated by retrogression or degeneration from 
other more highly organized ancestors, similar, but provided 
with a fully developed chorda. 
If we now suppose the case that of the known Tunicata 
the Appendicularie and all those numerous Ascidia which 
possess a chorda in the larval period had by chance disap- 
peared without leaving any traces, we may ask, what proof 
should we then have of the retrograde metamorphosis of the 
Tunicata? It might then easily happen that we should place 
them at the topmost stage of some lower but progressive series 
of animals, and therefore classify them incorrectly in the 
system. 
It will be seen from this that for correct classification caution 
and sober reason are above all necessary, especially in those 
cases where we have to do with the accurate determination of 
the relationships of an isolated group, with the development 
and other essential peculiarities of which we are only imper- 
fectly acquainted. 
When we take all this into consideration it appears clearly 
that side by side with progressive phylogenetic serves there are 
also retrogressive series and groups, which, in spite of fre- 
quently very divergent structure, may nevertheless not unfre- 
quently be very nearly allied. 
Darwin’s law of development justifies the assumption that 
among the many partly progressive, partly retrogressive 
series there must certainly exist one progressive develop- 
mental series, which commences below with a perfectly simple 
plastid and terminates above in man; but to decide which of 
the numerous branches and twigs of the genealogical tree of 
animals are truly progressive, or which of them owe their 
existence to a retrogressive phylogenesis, can only become 
possible in course of time, and then with any certainty, or at 
any rate with probability, only upon the foundation of embryo- 
logical and morphological investigations. So long, however, 
