T. Davidson — What is a Brachiopod ? 
269 
iferous, 2 Permian, 2 Triassic, 11 Jurassic, 5 Cretaceous, 3 Tertiary, 
and 9 in the recent periods. 1 But what wonderful changes liave 
beeil operating during the incalculable number of ages in which the 
creation (?) and extinction of a large number of genera and thousands 
of species bave 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 re- 
production. 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 Bracliiopods. The result might turn out very unfavourable to 
the views which I hold ; if so, so mucli 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 epocli. My fear is whether the Brachiopoda have clianged 
enough. The absolute amount of dilference 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 diininished, 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 im- 
portant than a hundred literary reviews), might be written by one 
like yourself, and without very great labour.” 
In several subsequently written letters, Darwin reiterates bis 
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 These numbers must of course be considered provisional, see Table. 
