Book II. ANIMAL PATHOtOGY. ^9^ 



not all produced at the game 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 indivi- 

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



1948. Androgynous animals are of two kinds ; those where impregnatoin takes place 

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

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



1949. Gemmiparous animals are exemplified in the hydra or fresh water polypus, and 

 other zoophytes. 



1950. Hybridous animals. In the accomplishment of the important purpose of ge- 

 neration, it is observed, that in the season of love, 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 affected 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. 



1951. 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. Tims, 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 Muscow 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. 1771. 

 p. 318.) 



1952. 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 an- 

 other. Among quadrupeds and birds, those individuals of different species which have united, have been con- 

 fined 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 kind. As the impregnating fluid, in such situ- 

 ations, is spread over the eggs after exclusion, a portion of it belonging to one species may have come in 

 contact with the impregnated eggs of another species, by the accidental movements of the water, and not 

 in consequence of any unnatural eftbrt. 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 indi- 

 viduals of the same species. 



1953. 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 preserve even in a domesticated state the intro- 

 duction 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 ; at tne same time there are few which are above 

 suspicion. 



Chap. V. 



Animal Pathology ; or the Duration^ Diseases, and Casualties of Animal Life. 



1 954. Each species of anim.al is destined, in the absence of disease and accidents, to 

 enjoy existence during a particular period. In no species, however, is this term absolutely 

 limited, as we find some individuals outliving others, by a considerable fraction of their 

 whole lifetime. In order to find the ordinary duration of life of any species, therefore, 

 we must take the average of the lives of a number of individuals, and rest satisfied with 

 the approximation to truth which can thus be obtained. There is little resemblance in 

 respect of longevity between the different classes, or even species of animals. There is no 

 peculiar structure, by which long-lived species may be distinguislied from those that are 

 short-lived. Many species whose structure is complicated, live but for a few years, as 

 the rabbit, while some of the testaceous mollusca, with more simple organization, have 

 a more extended existence. If longevity is not influenced by structure, neither is it 

 modified by the size of the species. While the horse, greatly larger than the dog, lives 

 to twice its age, man enjoys an existence three times longer than the former. 



1 955. The circumstances which regulate the term of existence in different species, ex- 

 hibit so many peculiarities,, corresponding to each, that it is difficult to offer any general 

 observations on the subject. Health is precarious, and the origin of diseases generally 

 involved in obscurity. The condition of the organs of respiration and digestion, 

 however, appears so intimately connected with the comfortable continuance of life, and 



IT 1 



