BIOLOGICAL PALAEONTOLOGY 117 



slow evolution, by which the selected types become 

 gradually improved and are progressively endowed with 

 those features that characterize the acmaic stage. 



At maturity the individual has reached the forefront 

 of its racial procession, and may progress a short 

 distance beyond it. After a varying period of march- 

 ing with the van, it tends, in typical cases, to slacken 

 progress. But the race streams forward, so that the 

 individual is left behind. Old age sets in ; even if the 

 attainments of maturity are kept, they soon prove to be 

 " behind the times." Once again the individual is in the 

 rear of the procession, but this time vitality is running 

 low ; efforts to " catch up " must end in failure. Various 

 physical and mental qualities develop which are dis- 

 advantageous, almost without exception. Increase in 

 bulk or sclerotic mineralization, often accompanied by 

 degeneration of important organs (such as teeth), hasten 

 and ensure the coming dissolution. But it is by no 

 means rare to find that a strange reversion to tendencies 

 reminiscent of early youth makes an inept appearance 

 in old age. " Second-childhood " affords a grotesque 

 and ponderous imitation of infancy, developed in com- 

 bination with the worn-out aftermath of maturity. It 

 seems to represent a final and despairing effort to regain 

 lost ground by a new start. Like the final movement of 

 Tschai'kowsky's sixth Symphony, it marks a last, hope- 

 less struggle before death brings peace and oblivion. 



The catagenetic stages of phylogeny afford an almost 

 startling parallel. The members of a phylogerontic 

 stock are " born old," and show various modifications 

 that are familiar in the senescent stages of individual 

 life. Some types, such as the Proboscidea, grow un- 

 wieldy and, by reduction in the rate of reproduction, 

 concentrate too much of the racial life into each 

 individual body. Others, such as the Echinoidea (and 



