﻿T. Davidson — What is a Brachiopod ? 269 



iferous, 2 Permian, 2 Triassic, 11 Jurassic, 5 Cretaceous, 3 Tertiary, 

 and 9 in the recent periods.' 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 re- 

 production. These remarks lead me to give some extracts from a 

 letter which I received from Dai'win 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 reall}' 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 Avhether 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 joresent 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 form^jtions 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 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. 



^ These numbers must of course be considered provisional, see Table. 



