56 
be more resemblance between sucb related species than sub- 
sequently. The investigations into the gradual growth oi 
embryos before birth show us that facts fully answer to this 
expectation. The differences between the members of the 
same class are slowly built up by the diverse development of 
forms at first utterly undistmguishable ; and the more nearly 
allied the members are, the later do the differences between 
111 The remaining test belonging to the head of present charac- 
teristics is one of an entirely different kind, which affords a 
natural transition to the next division concerning distribution 
in space. If the effect of natural selection upon species ex- 
posed to it be to preserve and perpetuate their most improved 
forms, it follows at once that in those places where natural 
selection is carried on most vigorously, there should the species 
be most improved. The severity of the selection depends 
mainly upon the amount of competition to which each living 
being is exposed; clearly, then, in wide-spread areas, where 
there are a large number both of races and individuals 
struggling together, it was to be expected that both im- 
provement and extinction should go on most rapidly ; m con- 
fined and isolated areas, where the races and individuals are 
fewer, it was to be expected that both these processes would 
go on much more slowly. And precisely so we find it. Iso- 
lated localities— as islands, fresh-water lakes, caves, &c.— are 
ever found to present the greatest number of peculiar forms, 
often so resembling bygone types as to receive the name ot 
“ living fossils.” While, if the. comparative improved condition 
of the species generally be inquired after, it needs but to put 
the flora and fauna of an isolated and extended area into actual 
competition, the result speaks for itself. The species from the 
latter, if introduced into the former, speedily supplant and ex- 
tinguish the greater part of them, while those from, tne former 
are altogether unable to retaliate if transferred to the latter. 
We come now to the second division of tests of consistency, 
those, namely, which concern distribution in space — tests 
perhaps the severest of any to which the hypothesis is sub- 
ject. Darwinism supposes that every species of a genus has 
descended from an original single species ; that every such 
representative species in each order has descended m like 
manner from one original, and so on. But these species and 
genera are scattered in all directions over the face of the 
globe. It is incumbent on the upholders of Darwinism to 
show, then, (1) how the original representative species could have 
become so distributed as that their varied descendants should 
appear in the places they now do ; and (2) that the systematic 
