NATURE OF REPRODUCTION. 



and the cancellated tissue becoming more open in texture. There is a 

 notable difference in the quantities of oxygen and carbonic acid inspired 

 and exhaled at different ages. The muscles, also, if examined after the 

 lapse of some years, are found to be less irritable than formerly, owing 

 to a slow, but steady and permanent deviation in their intimate con- 

 stitution. 



The vital properties of the organs, therefore, change with their vary- 

 ing structure ; and a time comes at last when they are perceptibly less 

 capable of performing their original functions than before. The very 

 exercise of the vital powers is inseparably connected with the subse- 

 quent alteration of the organs employed in them ; and the functions of 

 life, instead of remaining indefinitely the same, pass through a series 

 of successive changes, which finally terminate in their complete cessa- 

 tion. 



The history of a living animal or plant is, therefore, a history of suc- 

 cessive epochs or phases of existence, in each of which the structure 

 and functions of the body differ more or less from those in every other. 

 The organized being has a definite term of life, through which it passes 

 by the operation of an invariable law, and which, at some regularly 

 appointed time, comes to an end. The plant germinates, grows, blos- 

 soms, bears fruit, withers, and decays. The animal is born, nourished, 

 and brought to maturity, after which he retrogrades and dies. The 

 very commencement of existence, by leading through its successive 

 intermediate stages, conducts at last necessarily to its own termination. 



But while individual organisms are constantly perishing and disap- 

 pearing from the stage, the particular kind, or species, remains in exist- 

 ence, without any important change in the appearance of the organized 

 forms belonging to it. The horse and the ox, the oak and the pine, the 

 different kinds of wild and domesticated animals, even the different 

 races of man himself, have remained without any essential alteration 

 since the earliest historical epochs. Yet during this period innumer- 

 able individuals, belonging to each species or race, have lived through 

 their natural term and successively passed out of existence. A species 

 may therefore be regarded as a type or class of organized beings, in 

 which the particular forms composing it die off and disappear, but which 

 nevertheless repeats itself from year to year, and maintains its ranks 

 constantly full by the regular accession of new individuals. This pro- 

 cess, by which new organisms make their appearance, to take the place 

 of those which are destroyed, is known as the process of reproduction. 

 The first important topic, in the study of reproduction, is that of the 

 conditions necessary for its accomplishment. 



Reproduction by Generation. It is well known that, as a rule, in 

 the reproduction of any particular kind of living organism, the young 

 animals or plants are produced, directly or indirectly, from the bodies 

 of the elder. The relation between the two is that of parents and 

 progeny. The progeny, accordingly, owes its existence to an act of 

 generation; and the new organisms, thus generated, become in turn the 



