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Charles Darwin begins his book Origin of the Species (1859), in which he argues that species evolve as a result of variations caused by natural selection, with a chapter about variations that have occurred not naturally but under domestication. There he writes

I may, without here entering on any details, state that, from geographical and other considerations, I think it highly probable that our domestic dogs have descended from several wild species.

The ancestry of the subspecies Canis lupus familiaris is a controversial topic, but I have not found any papers which argue that it descends from several wild species. When the view is that the subspecies descends from two separately domesticated strands of ancestral wolf - East Eurasian and West Eurasian - the strands are not, as far as I am aware, referred to as separate species, but even if they were, two is not "several".

I appreciate that in a book on natural selection Darwin does not wish to spend more than a chapter on selection under domestication, but does he explain elsewhere his detailed reasons for his conclusion?

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"Does (Darwin) explain elsewhere his detailed reasons for his conclusion?"

Yes. After Origin of the Species, he wrote a whole book, The Variation of Animals and Plants under Domestication (1868), which begins with Chapter 1.I on "Domestic Dogs and Cats".

He writes that "we shall probably never be able to ascertain their origin with certainty" and then gives detailed sources for both sides of the argument. It's Pallas against De Blainville.

Firstly, the great difference between the several breeds; but this will appear of comparatively little weight, after we shall have seen how great are the differences between the several races of various domesticated animals which certainly have descended from a single parent-form. Secondly, the more important fact, that, at the most anciently known historical periods, several breeds of the dog existed, very unlike each other, and closely resembling or identical with breeds still alive (...)

(A)t a period between four and five thousand years ago, various breeds, viz. pariah dogs, greyhounds, common hounds, mastiffs, house-dogs, lapdogs, and turnspits, existed, more or less closely resembling our present breeds (...)

As long as man was believed to have existed on this earth only about 6000 years, this fact of the great diversity of the breeds at so early a period was an argument of much weight that they had proceeded from several wild sources, for there would not have been sufficient time for their divergence and modification. But now that we know, from the discovery of flint tools embedded with the remains of extinct animals in districts which have since undergone great geographical changes, that man has existed for an incomparably longer period, and bearing in mind that the most barbarous nations possess domestic dogs, the argument from insufficient time falls away greatly in value (...)

The existence of a single race, remarkably constant in form during the whole Neolithic period, is an interesting fact in contrast with what we see of the changes which the races underwent during the period of the successive Egyptian monuments, and in contrast with our existing dogs. The character of this animal during the Neolithic period, as given by Rutimeyer, supports De Blainville's view that our varieties have descended from an unknown and extinct form.

At this point it's not looking good for Pallas. But Darwin continues:

(W)e should not forget that we know nothing with respect to the antiquity of man in the warmer parts of the world. The succession of the different kinds of dogs in Switzerland and Denmark is thought to be due to the immigration of conquering tribes bringing with them their dogs; and this view accords with the belief that different wild canine animals were domesticated in different regions (...)

The main argument in favour of the several breeds of the dog being the descendants of distinct wild stocks, is their resemblance in various countries to distinct species still existing there.

Moreover,

(s)everal canine species evince no strong repugnance or inability to breed under confinement; and the incapacity to breed under confinement is one of the commonest bars to domestication. Lastly, savages set the highest value (...) on dogs: even half-tamed animals are highly useful to them: the Indians of North America cross their half-wild dogs with wolves, and thus render them even wilder than before, but bolder: the savages of Guiana catch and partially tame and use the whelps of two wild species of Canis, as do the savages of Australia those of the wild Dingo. Mr. Philip King informs me that he once trained a wild Dingo puppy to drive cattle, and found it very useful. From these several considerations we see that there is no difficulty in believing that man might have domesticated various canine species in different countries. It would indeed have been a strange fact if one species alone had been domesticated throughout the world (...)

If any wild canine species had distinctly exhibited the tan-coloured spots over the eyes, it might have been argued that this was the parent-form of nearly all our domestic races. But after looking at many coloured plates, and through the whole collection of skins in the British Museum, I can find no species thus marked. It is no doubt possible that some extinct species was thus coloured (...)

The belief that our dogs are descended from wolves, jackals, South American Canidae, and other species, suggests a far more important difficulty. These animals in their undomesticated state, judging from a widely-spread analogy, would have been in some degree sterile if intercrossed; and such sterility will be admitted as almost certain by all those who believe that the lessened fertility of crossed forms is an infallible criterion of specific distinctness. Anyhow these animals keep distinct in the countries which they inhabit in common. On the other hand, all domestic dogs, which are here supposed to be descended from several distinct species, are, as far as is known, mutually fertile together (...)

Pallas assumes (...) that a long course of domestication eliminates that sterility which the parent-species would have exhibited if only lately captured; no distinct facts are recorded in support of this hypothesis; but the evidence seems to me so strong (independently of the evidence derived from other domesticated animals) in favour of our domestic dogs having descended from several wild stocks, that I am inclined to admit the truth of this hypothesis.

There is another and closely allied difficulty consequent on the doctrine of the descent of our domestic dogs from several wild species, namely, that they do not seem to be perfectly fertile with their supposed parents. But the experiment has not been quite fairly tried; the Hungarian dog, for instance, which in external appearance so closely resembles the European wolf, ought to be crossed with this wolf: and the pariah dogs of India with Indian wolves and jackals; and so in other cases. That the sterility is very slight between certain dogs and wolves and other Canidae is shown by savages taking the trouble to cross them.

Concluding:

Notwithstanding the difficulties in regard to fertility given in the last two paragraphs, when we reflect on the inherent improbability of man having domesticated throughout the world one single species alone of so widely distributed, so easily tamed, and so useful a group as the Canidae; when we reflect on the extreme antiquity of the different breeds; and especially when we reflect on the close similarity, both in external structure and habits, between the domestic dogs of various countries and the wild species still inhabiting these same countries, the balance of evidence is strongly in favour of the multiple origin of our dogs.

In short the main support for multispecific origin is that different breeds of the domestic dog, differing from each other in many physical respects, are very ancient and they are similar to wild species in the same geographical areas.

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It seems that this was based on arguments in 1780 by Pyotr Simon Pallas that most domestic animals had descended from two or more aboriginal species that had since become mixed by interbreeding. Here's a brief summary in a letter Darwin wrote to Charles Lyell:

The doctrine of Pallas applied to our domestic breeds, we will say to Dogs, is as follows. He believes that man tamed a wolf in one country, the jackall in another, the fox in another & so on with other species. He does not suppose that these wild species were more variable than others. He hypothetically supposes that when long domesticated they lose their tendency to sterility when crossed with the other domesticated species; & by their crossing when domesticated he believes that all our domestic races have originated. He or his followers further believe that the crossing gives as a tendency to new characters to arise, & this is not a little hypothetical. The whole doctrine is very hypothetical. Yet I certainly believe (but cannot here give reasons) that the American domestic dogs have descended from at least 3 or 4 aboriginally distinct species, & that European dogs probably from several other species. There truly is, as you say, “an uncomfortable vague-ness” in whole doctrine.

--Letter to Charles Lyell, 25 October 1859

Lyell argues with Darwin on this in several letters, e.g. here and here.

Another letter shows a little more of his thinking:

By my theory, all dogs, wolves, foxes, jackalls, &c. have descended from some one very ancient species. The passage you allude to refers only to the amount of modification which our domestic dogs have undergone under domestication. I do not believe that the whole amount of difference in domestic dogs has been produced under domestication, but that part of difference is due to their having descended from several wild species. It is a distinct question whether these wild species have descended from one aboriginal stock as I believe has been the case.

--Letter to Caroline Sarah Wedgwood, after 21 November 1859

I don't know anything about Pallas, and as far as a cursory search shows me, he didn't write in English, so I can't say more about Pallas's reasoning. It seems that Darwin's argument for multiple origins boiled down to his gut feeling that dogs are too variable to have originated from a single species, but he readily admits that this isn't a strong argument, and seems reluctant to strongly defend it.

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  • $\begingroup$ Thanks for this. "(Pallas's) whole doctrine is very hypothetical. Yet I certainly believe (but cannot here give reasons) (...)" leaves it open as to whether he thought he had a strong intellectual argument or was going by gut feeling. I wonder what his personal involvement was with dogs. It's known he studied and bred pigeons, became very knowledgeable about their variation, and discussed them with expert breeders $\endgroup$ – user40471 Mar 6 '18 at 14:38
  • $\begingroup$ "I most heartily subscribe to what you say about the qualities of Dogs: I have one whom I love with all my heart." --Darwin, to unknown recipient $\endgroup$ – iayork Mar 6 '18 at 15:18
  • $\begingroup$ I wonder what the "rather old book" was that Darwin's unknown correspondent had referred to. $\endgroup$ – user40471 Mar 6 '18 at 15:51

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