Geographical Distribution of Crustacea. 13 
The species of highest rank among the Brachyura, Macroura, 
Isopoda, and Amphipoda, the four principal types in the above, 
belong to the extra-torrid zones; and in. subordinate groups or 
families, it is often true that the genera of superior grade are ex- 
tra-torrid, in contrast with the others which are torrid genera. 
Higher groups, characteristic of the colder regions, sometimes 
show degradation among those species of the group that are trop- 
ical; and the tropical sections also may continue the line of 
degradation by an extension again into the colder seas. 
s we descend in the scale of Crustacea from the Podoph- 
thalmia to the Tetradecapoda, the number of cold-water species 
increases, becoming in the latter group, three times greater 
than the warm-water species. It is an important fact, neverthe- 
less, that this increase of cold-water species is still no mark of 
degradation ; the particular facts that have been discussed, lead- 
ing to a very different conclusion. Other principles follow. 
These are— 
First, that the two types, the Decapodan and Tetradecapodan, 
are distinct types, to be independently considered, and not parts 
of a series or chain of species—a fact illustrated in the chapter 
on the Classification of Crustacea. 
Second, that the preponderance of cold-water species is the re- 
verse of what must have been true in the earlier geological epochs, 
when the oceans had a somewhat higher temperature ; or were to 
a large extent tropical. 
Third, that the progress of creation as regards Crustacea, has 
ended not where it begun, in multiplying the species of warmer 
waters and giving them there their superior developments, but in 
carrying species to a higher perfection in the colder regions of the 
Oceans. A preponderance of species in the warmer seas 1s per- 
haps to be expected, since warm waters have prevailed even more 
largely than now in earlier epochs. But it would seem, that the 
introduction of the higher grades of Crustacea required, not mere- 
ly the cooler waters of the present tropics, but even the still colder 
temperature of the Temperate zone, and therefore the 
condition of the globe. 
nv os athe isd tata aga eon 
Brachyura, ge “ = 
Macroura, : 57 
Tpopoda, io 
Amphipoda, ies : i ; sine 
Entomostraca, ‘ 188 
