[Footnote 9:  “Petrificata montium calcariorum non filii sed parentes sunt, cum omnis calx oriatur ab animalibus.”—­Systema Naturae, Ed. xii., t. iii., p. 154.  It must be recollected that Linnaeus included silex, as well as limestone, under the name of “calx,” and that he would probably have arranged Diatoms among animals, as part of “chaos.”  Ehrenberg quotes another even more pithy passage, which I have not been able to find in any edition of the Systema accessible to me:  “Sic lapides ab animalibus, nec vice versa.  Sic runes saxei non primaevi, sed temporis filiae.”]

So long as the Globigerinoe;, actually collected at the surface, have not been demonstrated to contain the elements of clay, the Challenger hypothesis, as I may term it, must be accepted with reserve and provisionally, but, at present, I cannot but think that it is more probable than any other suggestion which has been made.

Accepting it provisionally, we arrive at the remarkable result that all the chief known constituents of the crust of the earth may have formed part of living bodies; that they may be the “ash” of protoplasm; that the “rupes saxei” are not only "temporis," but “vitae filiae”; and, consequently, that the time during which life has been active on the globe may be indefinitely greater than the period, the commencement of which is marked by the oldest known rocks, whether fossiliferous or unfossiliferous.

And thus we are led to see where the solution of a great problem and apparent paradox of geology may lie.  Satisfactory evidence now exists that some animals in the existing world have been derived by a process of gradual modification from pre-existing forms.  It is undeniable, for example, that the evidence in favour of the derivation of the horse from the later tertiary Hipparion, and that of the Hipparion from Anchitherium, is as complete and cogent as such evidence can reasonably be expected to be; and the further investigations into the history of the tertiary mammalia are pushed, the greater is the accumulation of evidence having the same tendency.  So far from palaeontology lending no support to the doctrine of evolution—­as one sees constantly asserted—­that doctrine, if it had no other support, would have been irresistibly forced upon us by the palaeontological discoveries of the last twenty years.

If, however, the diverse forms of life which now exist have been produced by the modification of previously-existing less divergent forms, the recent and extinct species, taken as a whole, must fall into series which must converge as we go back in time.  Hence, if the period represented by the rocks is greater than, or co-extensive with, that during which life has existed, we ought, somewhere among the ancient formations, to arrive at the point to which all these series converge, or from which, in other words, they have diverged—­the primitive undifferentiated protoplasmic living things, whence the two great series of plants and animals have taken their departure.