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The number from the Permian formation, which completes the Palæozoic series, has not yet been computed, but they are comparatively few in number. Making a large allowance for synonyms, it will be seen that fully 3000 species are already known to have existed during the primary periods. It is also a remarkable fact that the Brachiopoda, so immensely abundant during the Cambrian, Silurian, Devonian, and Carboniferous periods, became apparently very much less numerous during the Permian and Triassic; while they again became abundant, although comparatively reduced in number, during the Jurassic and Cretaceous period. In the Tertiaries they had materially decreased in number, and are represented, at the present time, by about one hundred species. It has also been clearly ascertained that a certain number of genera and species passed from one system or formation into the one that followed it, as may be seen by a glance at the Table, in which the general distribution of the genera in time has been given with as much accuracy as the present state of our knowledge will admit. From this table it will be perceived that about 9 genera appeared for the first time in the Cambrian system, 52 in the Silurian, 21 in the Devonian, 7 Carbon-

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iferous, 2 Permian, 2 Triassic, 11 Jurassic, 5 Cretaceous, 3 Tertiary, and 9 in the recent periods.1 But what wonderful changes have been operating during the incalculable number of ages in which the creation (?) and extinction of a large number of genera and thousands of species have taken place. Some few only of the primordial, or first created genera, such as Lingula, Discina, and Crania, have fought their way and struggled for existence through the entire sequence of geological time. Many were destined to a comparatively ephemeral existence, while others had a greater or lesser prolongation of reproduction. These remarks lead me to give some extracts from a letter which I received from Darwin as far back as the 26th of April, 1861. In that letter, this eminent and admirable observer writes, "I do not know whether you have read my 'Origin of Species.' In that book I have made the remark, which I apprehend will be universally admitted, that as a whole, the fauna of any formation is intermediate in character between that of the formation above and below. But several really good judges have remarked to me how desirable it would be that this should be exemplified and worked out in some detail, and with some single group of beings. Now every one will admit that no one in the world could do this better than you with Brachiopods. The result might turn out very unfavourable to the views which I hold; if so, so much the better for those who are opposed to me. But I am inclined to suspect that on the whole it would be favourable to the notion of descent with modification. I can hardly doubt that many curious points would occur to any one thoroughly instructed in the subject, who could consider a group of beings under the point of view of descent with modification. All those forms which have come down from an ancient period very slightly modified ought, I think, to be omitted; and those forms alone considered which have undergone considerable change at each successive epoch. My fear is whether the Brachiopoda have changed enough. The absolute amount of difference of the forms in such groups at the opposite extremes of time ought to be considered, and how far the early forms are intermediate in character between those which appeared much later in time. The antiquity of a group is not really diminished, as some seem to think, because it has transmitted to the present day closely allied forms. Another point is how far the succession of each genus is unbroken from the first time it appeared to its extinction, with due allowance made for formations poor in fossils. I cannot but think that an important essay (far more important than a hundred literary reviews), might be written by one like yourself, and without very great labour."¹

In several subsequently written letters, Darwin reiterates his suggestions. I can assure you that I have not neglected a request coming from so eminent a quarter, but I am bound to state that I have found the subject beset with so many apparently inexplicable difficulties, that year after year has passed away without being able to trace the descent with modification among the Brachiopoda which the Darwinian doctrine requires.

¹ The full letter with important notes is published in Correspondence vol. 9, pp. 103-5. The editors noted (pp. 113-14, n. 6): "In the next-published volume of his monograph, which appeared in 1863, Davidson mentioned the possibility that fossil brachiopods might provide evidence for CD's view of species and varieties (Davidson 1851-86, 2: 212-13). He did not return to the question, however, until the final volume of his monograph, published shortly before his death. There Davidson quoted from CD's letter of 26 April 1861 and stated (ibid., 5: 387):

In several subsequent letters Darwin reiterated his suggestions; and, although I have not neglected a request coming from so eminent a quarter, I am bound to state that I have found the subject beset with so many apparently inexplicable difficulties that year after year has passed away without my being able to trace, in a satisfactory manner, the descent with modification among the Brachiopoda which the Darwinian doctrine requires.

Davidson stated that he none the less believed in some form of the doctrine of species transmutation."

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The imperfection (one due, I believe, to our slight acquaintance with the subject) in the geological record cannot in many cases be doubted, but we have no right to make capital out of unknown data. We must therefore deal with facts as we find them, and see how far they will bear upon the subject under examination. It may be quite true, that strata at great distances cannot be positively asserted to be strictly speaking absolutely contemporaneous, although they may contain the same animals.