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.