which up till then had been discovered, seemed to justify
the opinion that as time went on, forms had successively appeared with{135}
more and more complete segmentation and ossification of the backbone, which
in the earliest forms was (as it is in the lowest fishes now) a soft
continuous rod or notochord. Now, however, it is considered probable that
the soft back-boned Labyrinthodont Archegosaurus, was an immature or larval
form,[131] while Labyrinthodonts with completely developed vertebrae have
been found to exist amongst the very earliest forms yet discovered. The
same may be said regarding the eyes of the trilobites, some of the oldest
forms having been found as well furnished in that respect as the very last
of the group which has left its remains accessible to observation.

[Illustration: TRILOBITE.]

Such instances, however, as well as the way in which marked and special
forms (as the Pterodactyles, &c., before referred to) appear at once in and
similarly disappear from the geological record, are of course explicable on
the Darwinian theory, provided a sufficiently enormous amount of past time
be allowed. The alleged extreme, and probably great, imperfection of that
record may indeed be pleaded in excuse. But it _is_ an excuse.[132] {136}
Nor is it possible to deny the _a priori_ probability of the preservation
of at least a few _minutely transitional_ forms in some instances if
_every_ species without exception has arisen exclusively by such minute and
gradual transitions.

It remains, then, to turn to the other considerations with regard to the
relation of species to time: namely (1) as to the total amount of time
allowable by other sciences for organic evolution; and (2) the proportion
existing, on Darwinian principles, between the time anterior to the earlier
fossils, and the time since; as evidenced by the proportion between the
amount of evolutionary change during the latter epoch and that which must
have occurred anteriorly.

Sir William Thomson has lately[133] advanced arguments from three distinct
lines of inquiry, and agreeing in one approximate result. The three lines
of inquiry were--1. The action of the tides upon the earth's rotation. 2.
The probable length of time during which the sun has illuminated this
planet; and 3. The temperature of the interior of the earth. The result
arrived at by these investigations is a conclusion that the existing state
of things on the earth, life on the earth,