the fact that they differ little from any average
feather of birds in general, mark the contour feather as primitively
simple, and as the starting-point from which the highly elaborated
eye-painted tail feather has gradually evolved.

Applying these principles to the consideration of the Arachnida, we
arrive at the conclusion that the smaller and simpler Arachnids are not
the more primitive, but that the Acari or mites are, in fact, a
degenerate group. This was maintained by Lankester in 1878 (19), again
in 1881 (20); it was subsequently announced as a novelty by Claus in
1885 (21). Though the aquatic members of a class of animals are in some
instances derived from terrestrial forms, the usual transition is from
an aquatic ancestry to more recent land-living forms. There is no doubt,
from a consideration of the facts of structure, that the aquatic
water-breathing Arachnids, represented in the past by the Eurypterines
and to-day by the sole survivor Limulus, have preceded the terrestrial
air-breathing forms of that group. Hence we see at once that the
better-known Arachnida form a series, leading from Limulus-like aquatic
creatures through scorpions, spiders and harvest-men, to the degenerate
Acari or mites. The spiders are specialized and reduced in apparent
complexity, as compared with the scorpions, but they cannot be regarded
as degenerate since the concentration of structure which occurs in them
results in greater efficiency and power than are exhibited by the
scorpion. The determination of the relative degree of perfection of
organization attained by two animals compared is difficult when we
introduce, as seems inevitable, the question of efficiency and power,
and do not confine the question to the perfection of morphological
development. We have no measure of the degree of power manifested by
various animals--though it would be possible to arrive at some
conclusions as to how that "power" should be estimated. It is not
possible here to discuss that matter further. We must be content to
point out that it seems that the spiders, the pedipalps, and other large
Arachnids have not been derived from the scorpions directly, but have
independently developed from aquatic ancestors, and from one of these
independent groups--probably through the harvest-men from the
spiders--the Acari have finally resulted.

[Illustration: FIG. 31.--Diagram of a lateral view of a longitudinal
section of a scorpion.

After Beck, _Trans. Z