NATURE OF REPRODUCTION. 527 



leading through its successive intermediate stages, conducts at last 

 necessarily to its own termination. 



But while individual organisms are thus constantly perishing and 

 disappearing from the stage, the particular kind, or species, remains 

 in existence, apparently without any important change in the cha- 

 racter or 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 ever since the earliest 

 historical epochs. Yet during this period innumerable individuals, 

 belonging to each species or race, must 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 or structures composing it die off con- 

 stantly 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 process, by which new organ- 

 isms make their appearance, to take the place of those which are 

 destroyed, is known as the process of reproduction or generation. Let 

 us now see in what manner it is accomplished. 



It has always been known that, as a general rule in the process 

 of generation, the young animals or plants are produced directly 

 from the bodies of the elder. The relation between the two is that 

 of parents and progeny ; and the new organisms, thus generated, 

 become in turn the parents of others who succeed them. For this 

 reason wherever such plants or animals exist, they indicate the 

 previous existence of others belonging to the same species ; and if 

 by any accident the whole species should be destroyed in any par- 

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

 by the previous importation of others of the same kind. 



The commonest observation shows this to be true in regard to 

 those animals and plants with whose history we are more familiarly 

 acquainted. An opinion, however, has sometimes been maintained 

 that there are exceptions to this rule ; and that living beings may, 

 under certain circumstances, be produced from inanimate substances, 

 without any similar plants or animals having preceded them ; pre- 

 senting, accordingly, the singular phenomenon of a progeny without 

 parents. Such a production of organized bodies is known by the 

 name of spontaneous generation. It is believed by the large majority 

 of physiologists at the present day that no such spontaneous gene- 

 ration ever takes place; but that plants and animals are always 



