Origin of Species Variorum

Along this whole space of the Cordillera true glaciers do not now exist even at much more considerable heights. Farther south on both sides of the continent, from lat. 41° to the southernmost extremity, we have the clearest evidence of former glacial action, in huge boulders transported far from their parent source. ↑

9 blocks not present in 1859 1860 1861 1866; present in 1869 1872

From these several facts, namely from the glacial action having extended all round the northern and southern hemispheres— from the period having been in a geological sense recent in both hemispheres— from its having lasted in both during a great length of time, as may be inferred from the amount of work effected— and lastly from glaciers having recently descended to a low level along the whole line of the Cordillera, it formerly appeared to me that we could not avoid the conclusion that the temperature of the whole world had been simultaneously lowered during the Glacial period. But now Mr. Croll, in a series of admirable memoirs, has attempted to show that a glacial condition of climate is the result of various physical causes, brought into operation by an increase in the excentricity of the earths orbit. All these causes tend towards the same end; but the most powerful appears to be the influence of the excentricity of the orbit upon oceanic currents. It follows from Mr. Crolls researches, that cold periods regularly recur every ten or fifteen thousand years; but that at much longer intervals the cold, owing to certain contingencies, is extremely severe, and lasts for a great length of time. Mr. Croll believes that the last great Glacial period occurred about 240,000 years ago, and endured with slight alterations of climate for about 160,000 years. With respect to more ancient Glacial periods, several geologists are convinced from direct evidence that such occurred during the Miocene and Eocene formations, not to mention still more ancient formations. But in relation to our present subject, the most important result arrived at by Mr. Croll is, that whenever the northern hemisphere passes through a cold period, the temperature of the southern hemisphere is actually raised, with the winters rendered much milder, chiefly through changes in the direction of the ocean-currents. So conversely it is with the northern hemisphere, when the southern passes through a glacial period. These conclusions have, as we shall immediately see, a most important bearing on geographical distribution; but I will first give the facts, which demand an explanation.

We do not know that the Glacial epoch was strictly simultaneous at these several far distant points on opposite sides of the world. But we have good evidence in almost every case, that the epoch was included within the latest geological period. We have, also, excellent evidence, that it endured for an enormous time, as measured by years, at each point. The cold may have come on, or have ceased, earlier at one point of the globe than at another, but seeing that it endured for a long time at each, and that it was contemporaneous in a geological sense, it seems .. probable that it was, during a part at least of the period, actually simultaneous throughout the world. Without some distinct evidence to the contrary, we may at least admit as probable that the glacial action was simultaneous on the eastern and western sides of North America, in the Cordillera under the equatorial, tropical, and warmer temperate zones, and on both sides of the southern portion of the continent. If this be admitted, it is difficult to avoid believing that the temperature of the whole world was at this period simultaneously cooler. But it would suffice for my purpose, if the temperature was at the same time lower along certain broad belts of longitude.

On this view of the whole world, or at least of broad longitudinal belts, having been simultaneously colder from pole to pole, much light can be thrown on the present distribution of identical and allied species. In America, Dr. Hooker has shown that between forty and