i860.] PEDIGREE OF MAMMALIA. 341 



morphological or homological argument. You cut my throat, 

 and your own throat ; and I believe will live to be sorry for it. 

 So much for species. 



The striking extract which E. copied was your own writing ! ! 

 in a note to me, many long years ago which she copied and 

 sent to Mme. Sismondi ; and lately my aunt, in sorting her 



letters, found E.'s and returned them to her I have 



been of late shamefully idle, i.e. observing* instead of writing, 

 and how much better fun observing is than writing. 



Yours affectionately, 



C. DARWIN. 



C. Darwin to C. Lye/!. 



15 Marine Parade, Eastbourne, 



Sunday [September 23rd, 1860]. 



MY DEAR LVELL, I got your letter of the i8th just before 

 starting here. You speak of saving me trouble in answering. 

 Never think of this, for I look at every letter of yours as an 

 honour and pleasure, which is a pretty deal more than I can 

 say of some of the letters which I receive. I have now one 

 of 1 3 closely written folio pages to answer on species ! . . . . 



I have a very decided opinion that all mammals must have 

 descended from a single parent. Reflect on the multitude of 

 details, very many of them of extremely little importance to 

 their habits (as the number of bones of the head, &c., covering 

 of hair, identical embryological development, &c. &c.). Now 

 this large amount of similarity I must look at as certainly 

 due to inheritance from a common stock. I am aware that 

 some cases occur in which a similar or nearly similar organ 

 has been acquired by independent acts of natural selection. 

 But in most of such cases of these apparently so closely 

 similar organs, some important homological difference may be 

 detected. Please read p. 193, beginning, " The electric organs," 



