We have no reason for believing that the number of species on the earth at any former period was much less than at present; at all events the aquatic portion, with which the geologists have most acquaintance, was probably often as great or greater.  Now we know that there have been many complete changes of species, new sets of organisms have many times been introduced in place of old ones which have become extinct, so that the total amount which have existed on the earth from the earliest geological period must have borne about the same proportion to those now living as the whole human race who have lived and died upon the earth to the population at the present time....  Records of vast geological periods are entirely buried beneath the ocean ... beyond our reach.  Most of the gaps in the geological series may thus be filled up, and vast numbers of unknown and unimaginable animals which might help to elucidate the affinities of the numerous isolated groups which are a perpetual puzzle to the zoologist may be buried there, till future revolutions may raise them in turn above the water, to afford materials for the study of whatever race of intelligent beings may then have succeeded us.  These considerations must lead us to the conclusion that our knowledge of the whole series of the former inhabitants of the earth is necessarily most imperfect and fragmentary—­as much as our knowledge of the present organic world would be, were we forced to make our collections and observations only in spots equally limited in area and in number with those actually laid open for the collection of fossils....  The hypothesis of Prof.  Forbes is essentially one that assumes to a great extent the completeness of our knowledge of the whole series of organic beings which have existed on earth....  The hypothesis put forward in this paper depends in no degree upon the completeness of our knowledge of the former condition of the organic world, but takes what facts we have as fragments of a vast whole, and deduces from them something of the nature and proportion of that whole which we can never know in detail....

Another important series of facts, quite in accordance with, and even necessary deductions from, the law now developed, are those of rudimentary organs.  That these really do exist, and in most cases have no special function in the animal economy, is admitted by the first authorities in comparative anatomy.  The minute limbs hidden beneath the skin in many of the snake-like lizards, the anal hooks of the boa constrictor, the complete series of jointed finger-bones in the paddle of the manatee and the whale, are a few of the most familiar instances.  In botany a similar class of facts has been long recognised.  Abortive stamens, rudimentary floral envelope and undeveloped carpels are of the most frequent occurrence.  To every thoughtful naturalist the question must arise, What are these for?  What have they to do with