n the vertebrates, activity is concentrated in two pairs of
members only, and these organs perform functions which depend much less
strictly on their form.[60] The independence becomes complete in man,
whose hand is capable of any kind of work.

That, at least, is what we see. But behind what is seen there is what
may be surmised--two powers, immanent in life and originally
intermingled, which were bound to part company in course of growth.

To define these powers, we must consider, in the evolution both of the
arthropods and the vertebrates, the species which mark the culminating
point of each. How is this point to be determined? Here again, to aim at
geometrical precision will lead us astray. There is no single simple
sign by which we can recognize that one species is more advanced than
another on the same line of evolution. There are manifold characters,
that must be compared and weighed in each particular case, in order to
ascertain to what extent they are essential or accidental and how far
they must be taken into account.

It is unquestionable, for example, that _success_ is the most general
criterion of superiority, the two terms being, up to a certain point,
synonymous. By success must be understood, so far as the living being is
concerned, an aptitude to develop in the most diverse environments,
through the greatest possible variety of obstacles, so as to cover the
widest possible extent of ground. A species which claims the entire
earth for its domain is truly a dominating and consequently superior
species. Such is the human species, which represents the culminating
point of the evolution of the vertebrates. But such also are, in the
series of the articulate, the insects and in particular certain
hymenoptera. It has been said of the ants that, as man is lord of the
soil, they are lords of the sub-soil.

On the other hand, a group of species that has appeared late may be a
group of degenerates; but, for that, some special cause of retrogression
must have intervened. By right, this group should be superior to the
group from which it is derived, since it would correspond to a more
advanced stage of evolution. Now man is probably the latest comer of the
vertebrates;[61] and in the insect series no species is later than the
hymenoptera, unless it be the lepidoptera, which are probably
degenerates, living parasitically on flowering plants.

So, by different ways, we are led to the same conclusion. The evolution