Chapter XX: Letter 15: TO J.D. HOOKER. Down 1844

Thank you exceedingly for your long letter, and I am in truth ashamed of the time and trouble you have taken for me; but I must some day write again to you on the subject of your letter. I will only now observe that you have extended my remark on the range of species of shells into the range of genera or groups. Analogy from shells would only go so far, that if two or three species...were found to range from America to India, they would be found to extend through an unusual thickness of strata--say from the Upper Cretaceous to its lowest bed, or the Neocomian. Or you may reverse it and say those species which range throughout the whole Cretaceous, will have wide ranges: viz., from America through Europe to India (this is one actual case with shells in the Cretaceous period).