[178] Croll, in "Climate and Time," and in a note read before the British Association in 1876, takes an opposite view; but this is clearly contrary to the facts of sedimentation, which show a steady movement of _débris_ toward the south and south-west. While such considerations as those above referred to tend to reduce the practical importance of Mr. Croll's theory, on the other hand they tend to remove one of the greatest objections against it--namely, that founded on the necessity of supposing that glacial periods recur with astronomical regularity in geological time. They cannot do so if dependent on other causes inherent in the earth itself, and producing important movements of its crust. Sir Robert Ball has in a recent work very ingeniously improved this theory by showing that Croll was mistaken in assigning equal amounts of heat to the earth, as a whole, in the periods of greater and less eccentricity. This would tend to augment the effect of astronomical revolutions as causes of difference of temperature; but has no bearing on the more serious geological objections to the theory in question. A fatal objection, however, to Croll's theory, the force of which has been greatly increased by recent discoveries, is that the astronomical causes which he adduces would place the close of the last Glacial period at least 80,000 years ago, whereas it is now certainly known from geological facts that the close of the last Glacial period cannot be older than about an eighth or a tenth of that time. This difficulty seems to have caused the greater number of geologists, specially acquainted with the later geological periods, to regard this theory as quite inapplicable to the facts. We are thus obliged to fall back upon the old Lyellian theory of geographical changes, with such modifications as recent discoveries have rendered necessary. Taking this as our guide, we reach at once the important conclusion that the movements and distribution of animals and plants, however dependent on climate, altitude and depth, have, when regarded in connection with geological time, been primarily determined by those great movements of the crust of the earth which have established our islands, continents and ocean depths. These geographical changes have also in connection with animal and vegetable growth, deposition of sediments and volcanic ejections, fixed even the stations, soils and exposures of plants and animals. Thus, subject to those great astronomical laws which regulate the temperature of our planet as a whole, our attention may be restricted to the factors of physical geography itself. We must, however, carry with us the idea that though the great continents and the ocean depths may have been fixed throughout geological time, their relative elevations, and consequently their limits, have varied to a great extent, and are constantly changing.