Dear Sir

I beg to thank you for your kind note of the 23^d. which I received only yesterday morning, & happy would I feel were it in my power (which I regret it is not) to transmit the information required.¹ The subject is one to which I have myself paid much attention without being able to arrive at a satisfactory conclusion; for no shells can be more variable than the Brachiopoda, so much so that it seems very often difficult if not at times impossible to know where to draw a line of demarcation or to know what are the true limits of variation peculiar to individuals of a same species— The vast number of Brachiopoda that have passed through my hands or that have been examined by myself both in situ and in collections at home & abroad, have amply confirmed the opinion you have expressed that some forms even genera are more variable than others both in time and space. While preparing my monographs I have had much more difficulty in dealing with some than with other Species, and I have found that a variable Species is so in all localities however distant from which I have been able to examine Specimens and vice versa a form less variable is also so both in time & space. Thus the Spirifera Rostrata of Schlotheim has been found far spread both in Europe & America & every where has it presented that great variabili⁠⟨⁠ty⁠⟩⁠ of shape & character visible in any particular locality, being elongated or transverse more or less inflated smooth or ribbed & with or without a defined mesial fold & sinus—thus tempting naturalists to propose a number of names for what in reality constitutes but one single species all these extreme shapes being connected by every intermediate shade or passage to such an extent that it would be really impossible to trace a line of stoppage without violating nature which has granted to its forms a large range of variation. Ter. resupinata T. punctata and many hundred examples could be adduced in proof of this nature of things. (it is understood I only allude to full grown individuals) Now on the contrary certain other forms such as Rh. wrightii. and in particular many species of Lingula vary but little both in time & space—²

What causes these variations is difficult to explain but they are no doubt due to various causes depending on the same law which holds good with all the animal kingdom. That is to say that no two animals of a same species are exactly alike or cast in the same mould and t⁠⟨⁠hat⁠⟩⁠ the differences we perceive in our species are equally common to all types of life although our eyes or minds may not be able to weigh or appreciate them to an equal extent. Therefore variability is common to all species in a more or less degree dependent often on local circumstances such as habitat, climate, food etc—which forms races & true varieties. Thus for example some shores & bottoms seem to have been more favourable than others for the regular growth of individuals of a same species & here we have the cause why individuals of a same species are all more different in two such localities. Thus the Terebratella Menardi at Mans in France attained there its largest dimensions & most elegant shapes its ribs are sharp its shell thin and delicate, while at Farringdon in England where the same form occurs all the examples are stunted in growth the ribs more blunted & the shell more thickened & if we examine the bottoms on which they lived we find at once the cause to account for this remarkable difference. All these facts are as well or better known to yourself & to all naturalists & I repeat them merely to show that I cannot produce any ne⁠⟨⁠w⁠⟩⁠ explanation than that whi⁠⟨⁠ch⁠⟩⁠ has been already advocated, nor in repeated conversations I have had with several distinguished Zoologists have I been able to obtain any definite light on the subject of your note—

M^r. Bouchard Director of the Museum of Boulogne-sur-Mer³ has commissioned me to purchase if possible for him a memoir of yours, A Monograph of the Subclass Cirripeda 1854⁴ but he does not inform me where it is published, and as I have never seen the work would feel greatly obliged if you would kindly inform me where it can be had in town, as M^r. Bouchard is also writing a paper on the same subject and is most anxious to possess the work for reference etc—⁵

I am at present preparing a monograph of British Permian & Carboniferous species which will be illustrated by about 50 quarto plates⁶

I beg to remain Dear Sir Yours faithfully | Tho^s Davidson