For we have reason to believe that only a few species of a genus ever undergo change; the other species becoming utterly extinct and leaving no modified progeny.

Of the species which do change, only a few within the same country change at the same time; and all modifications are slowly effected.

I have also shown that the intermediate varieties which probably at first existed in the intermediate zones, would be liable to be supplanted by the allied forms on either hand; for the latter, from existing in greater numbers, would generally be modified and improved at a quicker rate than the intermediate varieties, which existed in lesser numbers; so that the intermediate varieties would, in the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting links, between the living and extinct inhabitants of the world, and at each successive period between the extinct and still older species, why is not every geological formation charged with such links?
Why does not every collection of fossil remains afford plain evidence of the gradation and mutation of the forms of life?
Although geological research has undoubtedly revealed the former existence of many links, bringing numerous forms of life much closer together, it does not yield the infinitely many fine gradations between past and present species required on the theory, and this is the most obvious of the many objections which may be urged against it.

Why, again, do whole groups of allied species appear, though this appearance is often false, to have come in suddenly on the successive geological stages?
Although we now know that organic beings appeared on this globe, at a period incalculably remote, long before the lowest bed of the Cambrian system was deposited, why do we not find beneath this system great piles of strata stored with the remains of the progenitors of the Cambrian fossils?
For on the theory, such strata must somewhere have been deposited at these ancient and utterly unknown epochs of the world's history.
I can answer these questions and objections only on the supposition that the geological record is far more imperfect than most geologists believe.

The number of specimens in all our museums is absolutely as nothing compared with the countless generations of countless species which have certainly existed.