and the E'lements in which they live. 371 
But, although we remove in this manner almost completely the 
circumstance of animals dwelling either in water or upon main 
land as influencing in any way our general classification of the 
animal kingdom, it were a great mistake to lose sight entirely of 
this most intimate relation among the natural secondary groups 
of animals under their different types. 
The value of these considerations has become more apparent, 
since the outlines of the leading divisions in the animal kingdom 
have been made in detail by allowing the results of embryology 
to have their due share of influence upon our classification; and 
the object of these remarks is chiefly to show that there is a 
universal relation throughout the animal kingdom between their 
structure and gradation and the elements in which they live; that, 
in all the four great types of the animal kingdom, the aquatic 
groups stand, in natural classification, lower than the terrestrial, 
and that this connection is so intimate as to extend even to the 
subdivisions, and so much so, that I have arrived at the conviction 
that in an otherwise well defined natural division, the aquatic 
tribes should be placed below the terrestrial ones; that even in 
narrowly circumscribed families the aquatic genera rank below 
the terrestrial, and that even in natural genera the aquatic species 
are inferior to the terrestrial ones. But before considering those 
minor divisions let us take a general glance at the four great types 
of the animal kingdom beginning with the Radiata. e5 
If we consider the type of Radiata as it is still cireumscribed in 
some of our most recent works upon the animal kingdom in gen- 
ral, we may fail to discover this intimate connection between 
their natural types and the media in which they live. But if we 
reduce the type of Radiata to those classes which I consider as 
alone truly representing that type, we shall be at once struck with 
the remarkable result, that all these animals are aquatic, nay, that, 
with one single exception, they are all marine. But before this 
ean be acknowledged, it must be shown that the type of Radiata 
should be reduced to the three classes of Polypi, Jelly-fishes and 
Echinoderms; and that, among Polypi, there are large numbers 
of animals now united which do not all belong to that class. 
The most extensive range acknowledged by some zoologists in 
the type of Radiata includes Infusoria with the Rotifera and also 
intestinal worms. . Without entering for the present into a full 
discussion of the natural character of all the animals which have 
been included in the class of Infusoria, I may limit my remarks 
to a few critical points, in order to show that the Polygastrica, 
and even the Rotifera cannot be ranked among Radiata. 
In the first place Rotifera constitute a particular group among 
Infusoria as Ehrenberg himself has acknowledged. . ‘They differ 
80 completely from the Polygastrica as to forbid entirely their 
union in a natural classification. The only question is whether 
