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do not occur until after long intervals, when changes of some kind in the physical conditions or through immigration have occurred; and individual differences or variations of the right nature, by which some of the inhabitants might be better fitted to their new places under the altered circumstances, might not at once occur. According to the standard of years we have no means of determining how long a period it takes to modify a species. Mr. Croll judging from the amount of heat-energy in the sun and from the date which he assigns to the last glacial epoch, estimates that only sixty million years have elapsed since the deposition of the first Cambrian formation. This appears a very short period for so many and such great mutations in the forms of life, as have certainly since occurred. It is admitted that many of the elements in the calculation are more or less doubtful, and Sir W. Thomson gives a wide margin to the possible age of the habitable world. But as we have seen, we cannot comprehend what the figures 60,000,000 really imply; and during this, or perhaps a longer roll of years, the land and the waters have everywhere teemed with living creatures, all exposed to the struggle for life and undergoing change.
On the Poorness of our Palæontological Collections .
Now let us turn to our richest geological museums, and what a paltry display we behold! That our .. collections are very imperfect is admitted by every one. The remark of that admirable palæontologist, .. Edward Forbes, should not be forgotten, namely, that numbers of our fossil species are known and named from single and often broken specimens, or from a few specimens collected on some one spot. Only a small portion of