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Darwin Correspondence Project

From Henry Denny   30 October 1844

Phill Hall Leeds

Oct 30th /44

Dr 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 Mr 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 | Dr Sir | Yours respectfully | Henry Denny Chas Darwin Esq | &c &c

CD annotations

crossed pencil
‘Worth examining how this is with land Birds living in distant Countries?’ added pencil
‘Land Birds of N America & Europe?’ added pencil


The British Association met in York in September 1844. Denny read a ‘Report of the progress of the investigation of the exotic Anoplura’, Report of the 14th meeting of the British Association for the Advancement of Science held at York in 1844, p. 392.
William Kirby (1825) described three species of insects which ‘assumed the characters of another tribe or genus’, one of which he named Pseudomorpha excrucians (pp. 98–101). Denny drew the plate which accompanies this description, but here the insect was called Heteromorpha excrucians (p. 109).


Has never heard of species of same genus [of parasites] being found on both birds and mammals, or different genera and species being found on animals in the domestic and wild states. Implications of this for relationship of aperea and guinea-pig.

Letter details

Letter no.
Henry Denny
Charles Robert Darwin
Sent from
Source of text
DAR 205.3: 273
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 785,” accessed on 18 January 2022,

Also published in The Correspondence of Charles Darwin, vol. 3