NATURE OF REPRODUCTION. 669 



parents of others which succeed them. For this reason, wherever such 

 plants or animals exist, they indicate the preceding existence of others 

 belonging to 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 previous 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 all the essential 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 popular experience it appears as a natural necessity. In reality 

 it is one of the most remarkable phenomena connected with the genera- 

 tive process ; and it indicates an unbroken connection of physiological 

 acts, extending through the entire lives of many different individuals. 

 Thus we know that the progeny of a fox will always be foxes ; and that 

 if we sow oats, it will be a crop of oats tiiat is produced in consequence. 

 Generation, accordingly, not only gives rise to new animals and plants, 

 or increases their number, but it also serves to continue indefinitely the 

 existence of the particular species, with all its characteristic marks and 

 qualities. 



Our idea, therefore, of a species, whether animal or vegetable, includes 

 two different elements, one of which is anatomical, the other physiologi- 

 cal. The anatomical character of a species consicts in the similarity of 

 form, size, and structure existing between all the individuals belonging 

 to it, and which we recognize at a glance j its physiological character 

 depends upon the fact, which has been learned by experience, that it 

 will reproduce itself, and that the different species in existence at any 

 one time remain distinct through an indefinite series of successive gene- 

 rations. 



It is not possible to say that the anatomical characters of species 

 have remained absolutely the same throughout all previous time, or 

 that they will continue to do so without limit in the future. The 

 existence of many fossil remains of animals and plants, different from 

 those which are known at the present day, shows that species are not 

 invariable and persistent through very long periods of time ; and that 

 they may either very gradually become so modified as to present a 

 different appearance, or else that they may entirely come to an end, 

 like the extinct mastodons and fossil horses of the United States, and 

 be replaced by others from a different localit}'. But in whatever way 

 the succession of species in different geological epochs be explained, it 

 is certain that at any one period their essential physiological characters 

 are those above described ; and that each species, by the process of 

 generative reproduction, remains distinct from the others which are 

 contemporary with it. 



But the production of young animals, similar in every respect to their 



