226 AGE, GROWTH, AND DEATH 



and more until the coil is complete. When we examine 

 a shell of that kind we see permanently before us the 

 various stages, both young and old, as recorded in 

 shell formation. And so too in the sea-urchin, and 

 in many of the common shell-fish, we find the double 

 record, of youth and old age, preserved permanently. 

 This has made it possible for Professor Hyatt and 

 for Professor Robert T. Jackson, who has adopted a 

 similar guiding principle, to bring new light into the 

 study of animal changes, and to attack the solution of 

 problems which without the aid of this senescent in- 

 terpretation, if I may so term it, would be utterly 

 impossible. This is an enticing subject, and I wish I 

 had both time and competency to dwell upon it. But 

 it is aside, as you see, from the main inquiries with 

 which we have been occupied, for our inquiries con- 

 cern chiefly the effect of cell-change upon the proper- 

 ties of the body, and the correlation of cell-change 

 with age. 



A natural branch of our topic is, however, that of 

 longevity, the duration of life. 1 Concerning this, we 

 have very little that is scientifically satisfactory that 

 we can present. We know, of course, as a funda- 

 mental principle, that every animal must live long 

 enough to reproduce its kind. Did that not occur, 

 the species would of course become extinct, and the 

 mere fact that the species is existing proves, of course, 



1 We are indebted to August Weissmann for raising the discussion of longevity 

 to the level of science. His essay, Ueber die Dauer des Lebens (Jena, 1882), is 

 by far the best on the subject known to me, and includes numerous data on the 

 longevity of animals. 



