ON ZGEAN INVERTEBRATA. 173 
with various combinations of colours, and the number of white species has 
greatly diminished. In the fourth, purple hues are frequent, and contrasts 
of colour common. In the third and second green and blue tints are met 
with, sometimes very vivid, but the gayest combinations of colour are seen in 
the littoral zone, as well as the most brilliant whites. 
The animals of Testacea and the Radiata of the higher zones are much more 
brilliantly coloured than those of the lower, where they are usually white, what- 
ever the hue of the shell may be. Thus the genus Trochus is an example 
of a group of forms mostly presenting the most brilliant hues both of shell 
and animal; but whilst the animals of such species as inhabit the littoral zone 
are gaily chequered with many vivid hues, those of the greater depth, though 
their shells are almost as brightly coloured as the coverings of their allies 
nearer the surface, have their animals for the most part of an uniform yellow 
or reddish hue, or else entirely white. 
The chief cause of this increase of intensity of colour as we ascend is doubt- 
less the increased amount of light above a certain depth. But the feeding 
grounds of the animals would appear to exert a modifying influence, and the 
reds and greens may be in many cases attributed to the abundance of nulli. 
pore and of the Caulerpa prolifera, a sea-weed of the most brilliant pea- 
green, the fronds of which the Mollusca of that colour, such as Nerita viridis, 
make their chosen residence. 
The eight regions in depth are the scene of incessant change. The death 
of the individuals of the several species inhabiting them, the continual acces- 
sion, deposition and sometimes washing away of sediment and coarser de- 
posits, the action of the secondary influences and the changes of elevation 
which appear to be periodically taking place in the eastern Mediterranean, 
are ever modifying their character, As each region shallows or deepens, its 
animal inhabitants must vary in specific associations, for the depression which 
may cause one species to dwindle away and die will cause another to multiply. 
The animals themselves, too, by their over-multiplication, appear to be the 
cause of their own specific destruction. As the influence of the nature of 
sea-bottom determines in a great measure the species present on that bottom, 
the multiplication of individuals dependent on the rapid reproduction of suc- 
cessive generations of Mollusca, &c. will of itself change the ground and 
render it unfit for the continuation of life in that locality until a new layer of 
sedimentary matter, uncharged with living organic contents, deposited on the 
bed formed by the exuviz of the exhausted species, forms a fresh soil for 
similar or other animals to thrive, attain their maximum, and from the same 
cause die off. This, I have reason to believe, is the case, from my observa- 
tions in the British as well as the Mediterranean seas. The geologist will 
see in it an explanation of the phenomenon of interstratification of fossilife- 
rous and non-fossiliferous beds. 
Every species has three maxima of development,—in depth, in geographic 
space, in time. In depth we find a species at first represented by few indi- 
viduals, which become more and more numerous until they reach a certain 
point, after which they again gradually diminish, and at length altogether 
disappear. So also in the geographic and geologic distribution of animals. 
Sometimes the genus to which the species belongs ceases with its disappear- 
ance, but not unfrequently a succession of similar species are kept up, repre- 
sentative as it were of each other. When there is such a representation the 
minimum of one species usually commences before that of which it is the 
representative has attained its correspondent minimum. Forms of repre- 
sentative species are similar, often only to be distinguished by critical exami- 
nation. When-a genus includes several groups of forms or subgenera, we 
