!6o SPREAD OF EVOLUTION. [1861. 



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 Geologi- 

 cal Succession. And I should like in this case to be per- 

 mitted to send you a copy of the new edition, just published, 

 in which I have added and corrected somewhat in Chapters 

 IX. and X. 



Pray excuse this long letter, and believe me, 



My dear Sir, yours very faithfully, 



C. Darwin. 



P.S. — I write so bad a hand that I have had this note 

 copied. 



C. Darwin to Thomas Davidson. 



Down, April 30, 1861. 

 My dear Sir, — I thank you warmly for your letter ; I did 

 not in the least know that you had attended to my work. I 

 assure you that the attention which you have paid to it, con- 

 sidering your knowledge and the philosophical tone of your 

 mind (for I well remember one remarkable letter you wrote 



