HISTORY OF ZOOLOGY. 29 



zebu; ibex (or wild buck) and she-goat; sheep and goats; dog and 

 jackal; dog and wolf : hare and rabbit (Lepus darwini); American 

 bison and domestic cattle; etc.; among birds, between different 

 species of finches and of grouse; mallard (Anas boschas) and the 

 pintail duck (Dafila acutd)' 3 the European goose and the Chinese 

 goose (Anser ferus and A. cygnoides}. Among the insects, 

 especially the Lepidoptera, the cases are many, but the resulting 

 eggs produce larvae of slight vital force only in the case of Actias 

 luna and A. isabellce. 



C. Fertility of Hybrids and Mongrels. Since many hybrids, 

 as the mule, have been known for thousands of years, the criterion 

 is, as it were, pushed back one stage ; if the infertility in cases of 

 crosses in many species is not immediately noticeable, yet it may 

 be apparent in the products of the cross. While the products of 

 the crossing of varieties, the ' mongrels/ always have a normal, 

 often an increased, fertility, the products of the crossing of 

 species, the hybrids, should always be sterile. But even this is 

 a rule, not a law. The mule (which only very rarely reproduces) 

 and many other hybrids are indeed sterile, but there are not a few 

 exceptions, although the number of experiments in reference to 

 this point is very small. Hybrids of hares and rabbits have con- 

 tinued fruitful J:or generations; the same is true of hybrids 

 obtained from the wild buck and the domesticated she-goat; from 

 Anser cygnoides and A. domesticus; from Salmo salvelinus and 

 S. fontinalis; Cyprinus carpio and Carassius vulgar is; Bombyx 

 cynthia and B. arrindia. 



D. Inbreeding. Even the second of the above statements, that 

 individuals of a species, provided they are sound, always reproduce 

 with one another, needs limitation. Breeders of animals have long 

 known the disastrous consequences of inbreeding that the repro- 

 ductive power is reduced even to sterility if, for breeding, 

 descendants of a single pair be continually chosen. Darwin has 

 collected not a few cases where undoubted members of the same 

 species have been completely sterile with one another; as certain 

 forms of primrose and other di- and tri-morphic species. Exam- 

 ples of the sterility of mongrels are known only in botany (certain 

 varieties of maize and mullein). 



Conditions Governing Fertility in Sexual Reproduction. 

 When we look over these facts it would seem as if continued 

 fertility in sexual reproduction were guaranteed by a not too con- 

 siderable diiference in the sexual products. Too great similarities, 

 as these exist in inbreeding, and too great differences, as in the 



