70 REPRODUCTION. 



they belong ; and the functions of life, instead of remaining indefi- 

 nitely the same, pass through a series of changes, which finally termi- 

 nate in their complete cessation. 



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

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

 and functions differ more or less from those in every other. The 

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

 according to an invariable law, and which, at some regularly appointed 

 time, comes to an end. The plant germinates, grows, blossoms, bears 

 fruit, withers, and decays. The animal is born, nourished, and brought 

 to maturity, after which he retrogrades and dies. The very commence- 

 ment of existence, by leading through its successive intermediate stages, 

 conducts at last necessarily to its termination. 



But while individual organisms are constantly perishing and disap- 

 pearing from the stage, the part'Vular kind, or species, remains in exist- 

 ence, without any important change in the forms belonging to it. The 

 horse and tho ox, the oak and tho pine, the numerous wild and domes- 

 ticated animals, as well as the different races of mankind, have remained 

 without essential alteration since the earliest historical epochs. Yet 

 during this period innumerable individuals, of 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 individuals composing it die off and 

 disappear, but which nevertheless repeats itself from year to year, and 

 maintains its ranks constantly full by. the continued accession of other 

 similar forms. This process, by which new organisms make their 

 appearance, in place of those which are destroyed, is 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, in the reproduction of animals or plants, the 

 young organisms are produced, as a rule, 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 parents of 

 others which succeed them. For this reason, wherever such plants or 

 animals exist, they indicate the preceding existence of the same species ; 

 and if by any accident the whole species should be destroyed in any 

 particular locality, no new individuals could be produced there, unless 

 by the importation of others of the same kind. 



The most prominent feature of generation, as a natural phenomenon, 

 is that the young animals or plants thus formed are of the same kind 

 with their parents. They reproduce the specific characters by which 

 their predecessors were distinguished ; and this takes place by a law 

 so universal that it seems almost a truism to state it. But this is only 

 because it has been so constantly a matter of observation, that in 



