lS6l.] DAVIDSON ON BRACHIOPODA. 367 



Spirifers, &c, from three palaeozoic stages, and arranged them 

 in single and branching lines, with horizontal lines marking 

 the formations (like the diagram in my book, if you know 

 it), and the result seemed to me very striking, though I was 

 too ignorant fully to appreciate the lines of affinities. I 

 longed to have had these shells engraved, as arranged by 

 Mr. Salter, and connected by dotted lines, and would have 

 gladly paid the expense : but I could not persuade Mr. Salter 

 to publish a little paper on the subject. I can hardly doubt 

 that many curious points would occur to any one thoroughly 

 instructed in the subject, who would consider a group of 

 beings under this 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 brachiopods 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 vaguely 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. I know 

 it is highly probable that you may not have leisure, or not 

 care for, or dislike the subject, but I trust to your kindness 

 to forgive me for making this suggestion. If by any extra- 

 ordinary good fortune you were inclined to take up this 

 notion, I would ask you to read my Chapter X. on Geo- 

 logical Succession. And I should like in this case to be 



