REJUVENESCENCE IN NATURE. 7 



generations is connected with the maturation of the 

 animal organism. 



Against this general relation, however, of the phe- 

 nomena of Rejuvenescence to progressive development, 

 may be raised the objection that the cases of Rejuvenes- 

 cence last mentioned, on which depend propagation of 

 natural bodies, are, in point of fact, very different in this 

 respect from those first spoken of, those occurring within 

 the cycle of the individual development ; in the one case 

 the aim of the Rejuvenescence would be a progress in the 

 development, while in the other it would be a mere repe- 

 tition of the like, as is distinctly demonstrated by the invari- 

 ability of species in the Animal and Vegetable kingdoms. 

 Yet this distinction vanishes when we test the gradations 

 of the cases, on both sides, and particularly if we rise above 

 the narrow field of vision of the present, in our examina- 

 tion of reproduction. The appearance of like alone being 

 repeated in nature, is removed in looking back from our 

 present stationary time into former epochs of the world. 

 There we find really the first beginnings of the species, 

 the genera, nay even of the orders and classes of the Vege- 

 table and Animal kingdoms ; we see at a glance how more 

 or less profound transformations were connected with the 

 appearance of the higher stages of the Organic Kingdoms, 

 so that genera and species of the ancient world disappeared 

 again, while new ones took their places. But through all 

 this change are expressed not mere accidental revolutions 

 of the earth, on the one hand of destroying, on the other 

 laying the basis of new soil for the flourishing progress 

 of organic life, but far rather definite laws, penetrating 

 into the very details of the development of organic life. 

 Thus, for example, among the Vertebrate series the Fishes 

 appear first, then the Amphibia, and, subsequent to both, 

 Birds and Mammalia. The Fishes of the first periods, as 

 fish the lowest of their class,* resemble in many respects 

 the Amphibia, vnore indeed than the fish subsequently 

 appearing, of higher orders, from the very circumstance 



* Agassiz. Poissonsfossiles, lut.roduction, xxx. 



