D^r Sir

It was my intention to have written you a line long before this, but first the York Meeting¹ —at which I had 8 plates to get ready to exhibit to the Nat Hist Section took up nearly all my time, since which various domestic occurrences have intervened to prevent me, until now— I feared by your last,² that you felt I had doubted your statements as to the exact locality of the Pediculi last sent, or implied a want of care, on your part.— Now nothing could be further from my wishes than to suspect either. But the appearance of the specimens from the Cavia Cobaia, was so strikingly different, from the Louse of the Domestic Guinea Pig, that, I thought an interchange of Specimens might accidentally have taken place in the way you alluded that by mixing in your bag, when out shooting By this means a species might be actually taken from a Bird or Quadruped, & yet not belong to it.— The specimens on a cursory inspection appeared to possess the exact similitude to the Genus Trinoton. The occurrence of which on any other that water Fowl I believe has never been noticed, at all events, I never heard of an instance in which species of the same Genus were found, some on Birds others on Mammals.

I am about to institute a rigid examination of the specimens, for on a second glance at them previous to going to York, I was struck at the singular appearance of them, in some points they looked like Trinoton, & yet there was a something which said they are not of that Genus—as if they wore a sort of disguise, somewhat like M^r Kirbys Heteromorpha.— If I am confirmed in this I should like to name the genus Pseudo-Morpha—but have a suspicion, the term is occupied already—. ³ If they turn up true Lice of the Aperea will they not tell against the Aperea & Guinea Pig—being identical. Zoologists are not all decided about this, yet— I cannot see why an animal should be infested by two peculiar parasites in a Domestic State, in England, France Germany Prussia &c—& by a totally different Genus & species, in its wild state.— We find the same Louse on the Spoonbill in Europe & Calcutta, the Gannet in Europe & Cape of Good Hope,—the Curlew Europe & India &c &c. What is your opinion concerning the Aperea being the origin of the Guinea Pig?— They do not agree in all points as for instance in the wild state but one young at a Birth & that seldom. In the domesticate state they breed freely— The colour again to me appears rather strange. In domestic Rabbits we have the wild colour as well as the varied but the Guinea pig never is of the same colour as the wild Aperea I believe?—

Would it not be worth while if I could find any one living in the country of the Aperea to examine more specimens for me, & send the results in Ship letters, if all the wild Aperea had the same Louse & not the Genus Gyropus at all, it would be ground for separating them I think— besides if I could find an Agent, I should like very much to see the Louse of the Capybara & Agouti & Coypou all of which are common in particular localities, I believe.—

Believe me | D^r Sir | Yours respectfully | Henry Denny Chas Darwin Esq | &c &c