29S SCIENCE OF AGRICULTURE. Part II. 



the lungs, to be again aerated. In their progress, they obtain the collected fluid of the 

 other absorbents, and, in the lungs, again prepare the whole for the use of the system. 

 Thus, during the continuance of life, the arteries supply the materials by which the sys- 

 tem is invigorated and enlarged, and oppose that tendency to decay, produced by the 

 influence of external objects. The process continues during the whole of life, new 

 matter is daily added, while part of the old and useless is abstracted. The addition is 

 greatest in early life, the abstraction is greatest in old age. 



1935. This continued system of addition and subtraction has led some to conclude, that a change in the 

 cor{)oreal identity of the body takes place repeatedly during the continuance of life, that none of the par- 

 ticles of which it consisted in youth, remain in its composition in old age. Some have considered the 

 change effected every three, others every seven years. This opinion, however, is rendered doubtful by 

 many well known facts. Letters marked on the skin by a variety of substances, frequently last for life. 

 There are some diseases, such as small-pox and measles, of which the constitution is only once susceptible ; 

 but it is observed to be liable to the attack of these diseases, at every period of human life. 



Sect. III. Of the reproductive System of Animals. 



1936. Animals are reproduced in consequence of the functions of certain organs, with 

 the exception of some of tlie very lowest in the scale. In those animals which possess 

 peculiar organs for the preparation of the germ or ovum, some are androgynous, (man- 

 woman) and either have the sexual organs incorporated, and capable of generating 

 without assistance, or the sexual organs are distinct, and the union of two individuals is 

 necessary for mutual impregnation : others have the sexual organs separate, and on dif- 

 ferent individuals. The young of such animals are either nourished at first by the store 

 of food in the egg, or by the circulating juices of the mother. Those species in which 

 the former arrangement prevails, are termed oviparous, while the term viviparous is 

 restricted to the latter. 



1937. In all animals it is the business of the female to prepare the ovum or germ, and 

 bring it to maturity. For this purpose, the germ is produced in the ovarium, farther 

 perfected in the uterus or matrix, and finally expelled from the system through the 

 vagina. The oflSce of the male is to impregnate the germ by means of the spermatic 

 fluid. This fluid is secreted in the testicles, transmitted by the spermatic ducts, and 

 finally conveyed by the external organ to its ultimate destination. 



1938. Among the viviparous animals, the reproductive organs present many points of 

 resemblance, and appear to be constructed according to a common model. It is other- 

 wise with the sexual organs of the oviparous tribes. They exhibit such remarkable dif- 

 ferences in the form and structure of all their organs, that it is impossible to collect them 

 into natural groups, or assign to them characters which they have in common. 



1 93!J. The manner in which the eggs of birds are impregnated by the male, has not been 

 satisfactorily determined. With the exception of the cicatricula, a female bird, in the ab- 

 sence of the male, can produce an egg. The conjunction of the sexes, however, is neces- 

 sary for the impregnation of the egg, and the effect is produced previous to the exclusion. 



1 940. In many kinds of fshes and reptiles, the yolks, after being furnished with their 

 glaire, are ejected from the body of the female, and the impregnating fluid in the male 

 is afterwards poured over them. Impregnation can be effected readily in such cases, by 

 the artificial application of the spermatic fluid. 



1941. Impregnation in insects appears to take place while the eggs pass a reservoir 

 containing the sperm, situated near the termination of the oviduct in the volva. 



1P42. The most simple mode of hatching is effected by the situation in which the eggs are placed by the 

 mother, after or during their exclusion. In this mode, a place is usually selected where the eggs will be 

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

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



1943. In the second mode, the mother, aided in some cases by the sire, forms a nest, in which she de- 



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

 atch their young in this manner. 



1944. In the last mode, the eggs are retained in the uterus, without any connexion, however, by circu- 

 lating 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 rana pipa, 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 ovoviviparous at another. 



1945. The young after being hatched^ are, in many cases, independent of their parent, and do not stand 

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

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

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

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



1946. 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 bronchiae ; both of which are absorbed, and disapjjear, 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 previous to appearing in its state of maturity. These 

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

 character according to the tribes or genera. 



1947. 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 



