294 SCIENCE OF AGRICULTURE. Part II. 



exposed to a suitable and uniform temperature, and where a convenient supply of food may be easily 

 obtained for the young animals. Such arrangements prevail in the insect tribe. 



1979. In the second mode, the mother, aided in some cases by the sire, forms a nest, in which she 

 deposits her eggs, and, sitting upon them, aids their hatching by the heat of her body. Birds in general 

 hatch their young in this manner. 



1980. In the third mode, the eggs are retained in the uterus, without any connection, however, by 

 circulating vessels, until the period when they are ready to be hatched, when egg and young are expelled 

 at the same time. This takes place in some sharks and Mollusca. The animals which exercise this last 

 kind of incubation are termed ovoviviparous. In the Rhnsi p^ipa, the eggs are deposited in a bag on the 

 back, where they are hatched, and where the young animals reside for some time after birth. Some 

 animals, as the aphis, are oviparous at one season, and ovov iparous at another. 



1981. The young, after being hatched, are, in many cases, independent of their parent, and do not stand 

 n need of any assistance : they are born in the midst of plenty, and have organs adapted to the supply of 



heir wants. Thus, many insects are hatched on, or within the very leaves which they are afterwards to 

 levour. In other cases, the young are able to follow their parents, and receive from them a supply of 

 -ippropriate food ; or, if unable to follow, their parents bring their food to the nests. 



1982. The changes which the young of oviparous animals undergo in passing from infancy to maturity 

 have long attracted the notice of the inquisitive observer. The egg of the frog is hatched in the water, 

 and the young animal spends in that element a part of its youth. While there it is furnished with a tail 

 and external bronchias; both of which are absorbed, and disappear, when it becomes an inhabitant of the 

 land. The infancy of the butterfly is spent in the caterpillar state, with organs of motion and mastication 

 which are peculiar to that period. It is destined to endure a second hatching, by becoming enveloped in a 

 covering, and suffering a transformation of parts previously to appearing in its state of maturity. These 

 metamorphoses of oviparous animals present an almost infinite variety of degrees of change, differing in 

 character according to the tribes or genera. 



1983. In birds, it is well known that one sexual union suffices for the production of impregnated eggs 

 during the period of laying. This is a case somewhat analogous to those quadrupeds which produce 

 several young at a birth with one impregnation, differing however, in the circumstance that the eggs are 

 not all produced at the same time, although they are afterwards hatched by the same incubation. In the 

 Aphides, or plant-lice, as they are called, one impregnation not only renders fertile the eggs of the 

 individual, but the animals produced from these, and the eggs of those again, unto the ninth generation. 



1 984. Androgynous animals are of two kinds ; those where impregnation takes place 

 by the mutual application of the sexual organs of two individuals ; and those where the 

 hermaphroditism is complete. The Mollusca exhibit examples of both kinds. 



1 985. GemmiparovA animals are exemplified in the -Hydra or fresh-water polypus, and 

 other zoophytes. 



1 986. Hybridous animals. In the accomplishment of the important purpose of ge- 

 neration, it is observed, that, in the season of desire, individuals of a particular species are 

 drawn together by mutual sympathy, and excited to action by a common propensity. 

 The produce of a conjunction between individuals of the same species partakes of the 

 characters common to the species, and exhibits in due time the characteristic marks of 

 puberty and fertility. In a natural state, the selective attribute of the procreative instinct 

 unerringly guides the individuals of a species towards each other, and a preventive 

 aversion turns them with disgust from those of another kind. In a domesticated state, 

 where numerous instincts are suppressed, and where others are fostered to excess, in- 

 dividuals belonging to different species are sometimes known to lay aside their natural 

 aversion, and to unite in the business of propagation. Instances of this kind occur 

 among quadrupeds, birds, and fishes, among viviparous and oviparous animals, where 

 impregnation takes place within, as well as when it is eflfected without, the body. The 

 product of such an unnatural union is termed a hybridous animal. The following cir- 

 cumstances appear to be connected with hybridous productions: 



1987. The parents must belong to the same natural genus or family. There are no exceptions to this 

 law. Where the species differ greatly in manners and structure, no constraints or habits of domestication 

 will force the unnatural union. On the other hand, sexual union sometimes takes place among indivi- 

 duals of nearly related species. Thus, among quadrupeds, the mule is the produce of the union of the 

 horse and the ass. The jackall and the wolf both breed with the dog. Among birds, the canary and 

 goldfinch breed together, the Muscovy and common duck, and the pheasant and hen. Among fishes, 

 the carp has been known to breed with the tench, the crusian, and even the trout. {Phil. Trans., 1/71, 

 p. 318.) 



1988. The parents must be in a confined or domesticated state. In all those hybridous productions which 

 have yet been obtained, there is no example of individuals of one species giving a sexual preference to 

 those of another. Among quadrupeds and birds, those individuals of different species which have united, 

 have been confined and excluded from all intercourse with those of their own kind. In the case of 

 hybridous fishes, the ponds in which they have been produced have been small and overstocked, and no 

 natural proportion observed between the males and females of the different kinds. As the impregnating 

 fluid, in such situations, is spread over the eggs afler exclusion, a portion of it belonging to one species 

 may have come in contact with the unimpregnated eggs of another species, by the accidental movements 

 of the water, and not in consequence of any unnatural effort. In all cases of this unnatural union among 

 birds or quadrupeds, a considerable degree of aversion is always exhibited, a circumstance which never 

 occurs among individuals of the same species. 



1989. The hybridous products are barren. The peculiar circumstances which are required to bring 

 about a sexual union between individuals of different species sufficiently account for the total absence of 

 hybridous productions in a wild state ; and, as if to prevent even in a domesticated state the introduction 

 and extension of spurious breeds, such hybridous animals, though in many cases disposed to sexual union, 

 are incapable of breeding. There are, indeed, some statements which render it probable that hybrid 

 animals have procreated with perfect ones j at the same time there are few which are above suspicion. 



