CONCLUSIONS FROM EXPERIMENTS 193 



ate cycles. In some of these the products of metabolism accumulate 

 only temporarily, and the period may cover only a few moments or 

 a few hours, while in others the fundamental features of organic 

 structure are concerned, and the period coincides with the life cycle. 



SENESCENCE AND REJUVENESCENCE IN EVOLUTION 



It is pertinent, at this time, at least to raise the question whether 

 the point of view and the conclusions reached from the study of 

 individuals have any value beyond the individual life cycle. Is 

 there any indication of the progressive senescence of species or 

 groups, and, if such senescence occurs, does it always lead to death, 

 i.e., extinction, or is rejuvenescence possible ? On the other hand, is 

 continued existence of a species without senescence possible ? 



Any answers to these questions must at the present time be little 

 more than guesses. It is possible, however, that the metabolic 

 substratum of the species may undergo very gradual progressive 

 changes of the same general character as those concerned in indi- 

 vidual senescence, but which are not entirely eliminated or com- 

 pensated during the periods of individual rejuvenescence, and it is 

 conceivable that under altered conditions regression might occur as 

 in individual rejuvenescence. It is also possible that the union of 

 two gametes from different lines of descent in gametic reproduction 

 may be an important factor in retarding or accelerating such 

 changes, if they occur. 



The records of paleontology are so fragmentary and our igno- 

 rance of the factors involved in the extinction or persistence of 

 species is so great that positive answers to these questions cannot be 

 looked for in that direction. Certainly many species have become 

 extinct in the course of geological time, but whether their extinction 

 has in any case been the result of a senescence we cannot deter- 

 mine. Decreasing numbers or decreasing size preceding extinction 

 may be due entirely to external conditions. But certain forms, such 

 for example as Limulus, the horseshoe crab, and the brachiopod 

 Lingula, have persisted practically unchanged from exceedingly 

 remote geological periods. Have such species not undergone senes- 

 cence, or has a rejuvenescence occurred somewhere, or perhaps 

 periodically, in the course of their descent ? 



