12 THE DEPTHS OF THE SEA. [CHAP. I. 
has‘yet been observed of one species passing through 
a series of inappreciable modifications into another. 
Every species appears to have an area of maximum 
development, and this has been called the metropolis 
of the species; and practically we must employ the 
same methods in investigating the laws of its distri- 
bution as if we still regarded it as having been 
specially created in its metropolis. 
It is the same in dealing with the law of represen- 
tation. Accepting an evolution doctrine, we should 
certainly regard closely allied or ‘representative’ 
species as having descended comparatively recently 
from a common ancestry, and as having diverged 
from one another under somewhat different conditions 
of life. It is possible that as our knowledge increases 
we may be able to trace the pedigree of our modern 
species, and some attempts have already been made 
to sketch out the main branches of the universal 
genealogical tree ;* but practically we must continue 
to accord a specific rank to forms which exhibit 
characters to which we have been in the habit of 
assigning specific value. 
«Every species has three maxima of develop- 
ment,—in depth, in geographic space, in time. I» 
depth, we find a species at first represented by few 
individuals, 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 
1 Ernst Haeckel, op. cit. 
