The only real answer to this objection is that the question is, not whether Croll's theory is the sole or even the principal cause of glacial periods, or able to influence them materially if the geographical conditions favour genial climates; but whether it has not a co-operating effect, when these conditions are such as to produce glaciation. It seems difficult to suppose that such contrasts of conditions as are pointed out by Sir R. Ball can have had no perceptible effect on climates; or that such close coincidences as are shown between the astronomical theory and geological facts, during the last glacial period, can be due to mere accident. Geology shows six phases of this period:--(1) a refrigeration coming on in the Pliocene; (2) its culmination in a first and most intense _maximum_; (3) a gradual return to a milder inter-glacial period; (4) a second refrigeration; (5) its culmination in a second _maximum_; (6) a second return to genial conditions, such as still prevail. Croll's theory shows six astronomical phases, corresponding to these six geological phases. Geology shows that each of its six phases involves several minor alternations of hot and cold; Croll shows that this must have been the case owing to the effects of the shorter cycles of precession, occurring during the long cycles of variations in eccentricity. Geology tells us that cold alone would not account for a glacial period; we must have heat to supply the evaporation which is condensed by the cold; Croll shows that with high eccentricity cold and long winters must have been accompanied by short and hot summers. And Sir R. Ball's recent calculations show that the argument is really very much stronger than Croll puts it. The duration of each of the phases of Croll's theory corresponds also, on the whole, remarkably well with that required for each phase of the geological record. They would average about 40,000 years each for Croll's phases, and a less time can hardly be allowed for the immense amount of geological work in the way of denudation and deposition, elevation and depression, and changes of fauna and flora which have occurred since the commencement of the great refrigeration in the late Pliocene. In fact the only reasonable doubt seems to be whether Croll's times are sufficient, and whether, as Lyell was inclined to think, the first and greatest glaciation must not be carried back to the extreme period of high eccentricity which occurred about 700,000 years ago.