Miscellaneous. 383 
foot of a dog and of a horse, the wing of a bat, and the fin of a por- 
poise are fundamentally identical ; that the long neck of the giraffe 
has the same and no more bones than the short one of the elephant; 
that the eggs of Surinam frogs hatch into tadpoles with as good tails 
for swimming as any of their kindred, although as tadpoles they 
never enter the water ; that the Guinea-pig is furnished with incisor 
teeth which it never uses, as it sheds them before birth; that em- 
bryos of Mammals and Birds have branchial slits and arteries running 
in loops, in imitation or reminiscence of the arrangement which is 
permanent in Fishes ; and that thousands of animals and plants have 
rudimentary organs which, at least in numerous cases, are wholly 
useless to their possessors, &c. Upon a derivative theory this mor- 
phological conformity is explained by community of descent; and 
it has not been explained in any other way. 
Naturalists are constantly speaking of “related species,” of the 
“affinity” of a genus or other group, and of “ family resemblance,” 
—vaguely conscious that these terms of kinship are something more 
than mere metaphors, but unaware of the grounds of their aptness. 
Mr. Darwin assures them that they have been talking derivative 
doctrine all their lives without knowing it. 
If it is difficult, and in some cases practically impossible, to fix the 
limits of species, it is still more so to fix those of genera ; and those 
of tribes and families are still less susceptible of exact natural cir- 
eumscription. Intermediate forms occur, connecting one group with 
another in a manner sadly perplexing to systematists, except to those 
who have ceased to expect absolute limitations in nature. All this 
blending could hardly fail to suggest a former material connexion 
among allied forms, such as that which a hypothesis of derivation 
demands. 
Here it would not be amiss to consider the general principle of 
gradation throughout organic nature,—a principle which answers in 
a general way to the law of continuity in the inorganic world, or rather 
is so analogous to it that both may be fairly expressed by the Leib- 
nitzian axion, Natura non agit saltatim. Asan axiom or philosophical 
principle, used to test model laws or hypotheses, this in strictness 
belongs only to physics. In the investigation of Nature at large, at 
least in the organic world, nobody would undertake to apply this 
principle as a test of the validity of any theory or supposed law. 
But naturalists of enlarged views will not fail to infer the principle 
from the phenomena they investigate,—to perceive that the rule 
holds, under due qualifications and altered forms, throughout the 
realm of Nature, although we do not suppose that Nature in the 
organic world makes no distinct steps, but only short and serial 
cela infinitely fine gradations, but no long leaps, or few of 
em. 
To glance at a few illustrations out of many that present them- 
selves. It would be thought that the distinction between the two 
organic kingdoms was broad and absolute. Plants and animals be- 
long to two very different categories, fulfil opposite offices, and, as to 
