1004 THE PHYSIOLOGY OF REPRODUCTION. 



The course of metabolism is such that it is self-limited, and even 

 if perfect conditions were supplied natural death would eventually 

 result. As a rule, however, death is, so to speak, accidental 

 rather than natural. This point of view was emphasized by Ray 

 Lankester in the distinction which he drew between specific 

 longevity and potential longevity. By specific longevity he des- 

 ignated the expectation of life at birth of a normal individual, 

 this expectation being determined by the interaction of two fac- 

 tors namely, the innate constitutional properties of the proto- 

 plasm and adverse environmental conditions. By potential lon- 

 gevity is meant the duration of life which might be expected 

 under an ideal environment. The average specific longevity, 

 reckoned from birth, is greatest in the most civilized communities 

 and reaches at present the length of forty-five to fifty years. 

 Quite probably the advances of medical knowledge, especially of 

 preventive medicine, may continue to increase the average of this 

 expectation of life. What the potential longevity of man would 

 be if protected from all accidents and disease cannot be stated 

 with any certainty, but on the basis of the exceptional cases of 

 longevity reported we may assume that it would exceed one 

 hundred years. The most authentic of the cases reported of 

 unusual longevity is that of Thomas Parr. An account of his 

 life and the results of a postmortem examination by Harvey 

 are given in Volume III of the " Philosophical Transactions of 

 the Royal Society of London." "He died after he had out- 

 lived nine princes, in the tenth year of the tenth of them, at 

 the age of one hundred and fifty-two years and nine months." 

 The immediate cause of his death was attributed to a change 

 of food and air and habits of life, as he was brought from Shrop- 

 shire to London, "where he fed high and drunk plentifully of the best 

 wines."* With reference to the phenomenon of senescense as a neces- 

 sary attribute of living matter, Weissmann has called attention to the 

 fact that inasmuch as the species continues to exist after the in- 

 dividual dies, we must believe that the protoplasm of the repro- 

 ductive elements is not subject to natural death, but has a self- 

 perpetuating metabolism which under proper conditions makes it 

 immortal. Weissmann f designates the protoplasm of the germ cells 

 as germ-plasm, that of the rest of the body as somatoplasm, and 

 inasmuch as the former continues to propagate itself indefinitely 

 under proper conditions, while the latter has a limited existence, he 

 concludes that originally protoplasm possessed the property of 



* A picture of Parr painted by van Dyck (1635) is exhibited in the Royal 

 Gallery, Dresden, No. 1032. 



t Weissmann, "Essays upon Heredity and Kindred Biological Prob- 

 lems"; also "Germ-plasm" in the "Contemporary Science Series." 



