It deserves notice, as showing how the capacity to breed sometimes goes by affinity, that the one native rodent of Paraguay, which there breeds FREELY and has yielded successive generations, is the Cavia aperea; and this animal is so closely allied to the guinea-pig, that it has been erroneously thought to be the parent form. (18/20. Rengger 'Saugethiere' etc. s. 276. On the parentage of the guinea-pig, see also Isid. Geoffroy St.- Hilaire 'Hist. Nat. Gen.' I sent to Mr. H. Denny of Leeds the lice which I collected from the wild aperea in La Plata, and he informs me that they belong to a genus distinct from those found on the guinea-pig. This is important evidence that the aperea is not the parent of the guinea-pig; and is worth giving, as some authors erroneously suppose that the guinea-pig since being domesticated has become sterile when crossed with the aperea.) In the Zoological Gardens, some rodents have coupled, but have never produced young; some have neither coupled nor bred; but a few have bred, as the porcupine more than once, the Barbary mouse, lemming, chinchilla, and agouti (Dasyprocta aguti) several times. This latter animal has also produced young in Paraguay, though they were born dead and ill-formed; but in Amazonia, according to Mr. Bates, it never breeds, though often kept tame about the houses. Nor does the paca (Coelogenys paca) breed there. The common hare when confined has, I believe, never bred in Europe; though, according to a recent statement, it has crossed with the rabbit. (18/21. Although the existence of the Leporides, as described by Dr. Broca ('Journal de Phys.' tome 2 page 370), has been positively denied, yet Dr. Pigeaux ('Annals and Mag. of Nat. Hist.' volume 20 1867 page 75) affirms that the hare and rabbit have produced hybrids.) I have never heard of the dormouse breeding in confinement. But squirrels offer a more curious case: with one exception, no species has bred in the Zoological Gardens, yet as many as fourteen individuals of S. palmarum were kept together during several years. The S. cinera has been seen to couple, but it did not produce young; nor has this species, when rendered extremely tame in its native country, North America, been ever known to breed. (18/22. 'Quadrupeds of North America' by Audubon and Bachman 1846 page 268.) At Lord Derby's menagerie squirrels of many kinds were kept in numbers, but Mr. Thompson, the superintendent, told me that none had ever bred there, or elsewhere as far as he knew. I have never heard of the English squirrel breeding in confinement. But the species which has bred more than once in the Zoological Gardens is the one which perhaps might have been least expected, namely, the flying squirrel (Sciuropterus volucella): it has, also, bred several times near Birmingham; but the female never produced more than two young at a birth, whereas in its native American home she bears from three to six young. (18/23. Loudon's 'Mag. of Nat. Hist.' volume 9 1836 page 571; Audubon and Bachman 'Quadrupeds of North America' page 221.)
Monkeys, in the nine-year Report from the Zoological Gardens, are stated to unite most freely, but during this period, though many individuals were kept, there were only seven births. I have heard of only one American monkey, the Ouistiti, breeding in Europe. (18/24. Flourens 'De l'Instinct' etc. 1845 page 88.) A Macacus, according to Flourens, bred in Paris; and more than one species of this genus has produced young in London, especially the Macacus rhesus, which everywhere shows a special capacity to breed under confinement. Hybrids have been produced both in Paris and London from this same genus. The Arabian baboon, or Cynocephalus hamadryas (18/25. See 'Annual Reports Zoolog. Soc.' 1855, 1858, 1863, 1864; 'Times' newspaper August 10, 1847; Flourens 'De l'Instinct' page 85.), and a Cercopithecus have bred in the Zoological Gardens, and the latter species at the Duke of Northumberland's. Several members of the family of Lemurs have produced hybrids in the Zoological Gardens.