of the Animal Kingdom. 317 
when the same naturalist referred the Vorticelle to the group 
Bryozoa merely from their external similarity, without at all 
taking into account the important differences in their structure 
and development; and the classifications of the Medusz at- 
tempted by Gegenbaur according to the presence or absence 
of a velum (Craspedota and Acraspeda), by Forbes * with 
exclusive reference to the marginal corpuscles (Steganoph- 
thalmata and Gymnophthalmata), and by Eschscholtz fT ac- 
cording to the position of the generative organs (Phanero- 
carpe and Cryptocarpe) are equally unsatisfactory. 
There is consequently no doubt that for the foundation of 
a truly natural or so-called phylogenetic classification the 
knowledge and consideration of the individual peculiarities is 
not sufficient, however important these peculiarities may be, 
or however early a period of development they may pertain to. 
The distinguished naturalist Fritz Miller, in his exceedingly 
valuable little work ‘ Fiir Darwin,’ has given us a perfectly 
new systematic grouping of the Crustacea according to one of 
the earliest embryonal characters, namely the segmentation 
and the mode of curvature of the embryo within the egg 
(Holoschista and Hemischista, Gasterotropa and Nototropa) ; 
but he thereby furnished a proof that the groups proposed by 
him were any thing rather than natural groups {. 
Among all properties of animals the biological conditions 
especially offer the least certainty in their classification, as 
even animals of admittedly different origin frequently live 
under exactly similar conditions, and in accordance with this 
not unfrequently also exhibit the same peculiarities. Thus, 
for example, the Cetacea and fishes live under exactly similar 
conditions, and therefore we find, in consequence of adapta- 
tion, not only the external form, but also the organs of motion, 
similarly formed, although, in fact, they are by no means 
nearly allied to one another. But even among the great 
marine Mammalia, according to recent investigations, the 
Sirenia prove to be essentially different from the true whales 
or Cetacea; and yet both forms of animals are externally so 
similar as to be confounded together, and they live under 
exactly the same conditions. 
* Forbes, ‘Monograph of the British Naked-eyed Medusee’ (London, 
1848). 
ji Hechecholta, ‘System der Acalephen’ (Berlin, 1829). 
{ [Leipzig, 1864. English translation under the title of ‘Facts and 
Arguments for Darwin’ (London, 1869). The classification here cited 
is given by Fritz Miller ironically, to show to what results the yin 3 
tion of certain authoritative dogmas would lead in the case of the Crus- 
tacea— TRANSL. ] 
