486 



NA rURE 



[March 23, 1899 



assumes llial the ^kaier, in fact, glides about on a narrow film 

 of water, the solid turning to water wherever the pressure is 

 most intense, and this water, continually forming under the 

 skate, resuming the solid form when relieved of pressure." 



J. JOLY. 

 Geological Laboratory, Trinity College, Dublin. 



Mammalian Longevity. 



The letter of Dr. Ainslie Hollis in Nature of January 5, 

 on "The Curve of Life," shows that the ratio existing between 

 the periods of maturity and the periods of after-life in various 

 mammals are capable of projection in a regular curve. This 

 led me to inquire if the ratios might not be capable of reduc- 

 tion to a general formula. This seems to be the case, the 

 statement being as follows : — 



The full term of life in a mammalian species is equal to 

 ten and a half timLS the period of maturity divided by the cube 

 root of the period (of maturity), that is 



1. t. 1. = •'*' ', or 10 5 ;< (p. m. ),J 

 v'(P- ni-) 

 in which f. t. I. is the full term of life, and p. m. the period of 

 maturity. 



By the full term of life is meant the period that the animal 

 would live, supposing that its existence were not shortened by 

 enemies, accidents, disease, starvation, overwork or nervous 

 strain ; and that it passed out of life by senile decay. This, of 

 course, simply represents an average. It is generally accepted 

 that the period of maturity is best measured by finding the age 

 at which the epiphyses are united to the skeleton. It seems to 

 be about from one and a half times to twice the period of 

 puberty : one and two-thirds and twice seem common propor- 

 tions. Man, for example, arrives at puberty at about fifteen, 

 and is mature at twenty-five ; the lion and tiger arrive at puberty 

 at three years, and are mature at six. 



A table is given below, showing the periods of maturity and 

 the full terms of life as obtained from the observations of 

 breeders, scientific men, iS:c. ; and, for purposes of comparison, 

 the full terms of life as calculated by the formula from the same 

 periods of maturity. It will he seen that the two agree as 

 closely as could be expected, especially when we bear in mind 

 the difficulty of fixing with precision the normal life of a 

 species, whose individual members will often die at widely 

 differing ages, from different causes. Hence the different 

 results obtained in many cases by different observers. 



The approximation of the results of observation and tin 

 formula will be noticed. Blaine on the horse, is from lli. 

 " Encyclop.T.'dia of Rural Sports." He seems to have .sludic 

 the subject of the horse's age very closely. He says: ". . . . 

 horse of five years may be comparatively considered as old as 

 man of twenty ; a horse of ten years, as a man of forty ; . 

 and of thirty-five years, as a man of ninety." Up to ten years 

 age, then, the horse counts one year i>roportionalely for ever;, 

 four of man, and as man's maturity takes place at iwenty-fiv. 

 this makes the horse's to occur at six and a quarter years. Tli 

 full term of life given as equivalent to a man of ninety, thiri\ 

 five years is almost identical with the result of the forniul 

 Darwin's observations on the elephant .ire from the "Origin " 

 Species," where he discusses the increase of animals. The oth. 

 references are from the works of various writers. It shoulil I 

 pointed out in connection with the dormouse, that Dr. .\insli 

 Hollis gave its full term of life as four years in Nature and fou 1 

 and a half in the Ltiiuel of January 21. 



Krnest D. Bei t.. 



The editor of Nature has kindly forwarded me .Mi 

 Ernest D. Bell's letter before publication. The formula there! ii 

 stated is interesting, as it confirms the opinion, given in m> 

 previous letter on the subject, that a relationship exists between 

 the duration of adolescence and theilength of a mammal's life. 

 Since the publication of the curve of life in Nature, I find 

 that the following domestic animals can be added to those 

 already given. They conform to the requirements of the curve 

 very closely, as may be seen : — 



ouinca-pig 



English grcyhoi 



Cat (Mivarl) 



Cat (Jennings) 



Hog 



Knglish hunter 



length of adolescence. 



6i 



Le n gth ofLife. 

 Observed Computed 



by cur ■ - 



6-7 years 7 years 



by form. 

 7 33 yen 



16-67 

 307 

 35-63 



The age at which growth ceases in man difTers considerably 

 in different individuals of the same race. Otto observed all the 

 epiphyses separate in the skeleton of a tiian, aged twenty-seven 

 years l.South's " Pathological Anatomy,'' p. 126 1. Such a skeleton 

 could not have completed its growth for another ten or twelve 

 years. The man, had he lived, might have truthfully posed as 

 a youth when he was on the verge of forty. I have in skia 

 grams observed a difference of upwards of three years in the 

 ages of different subjects, at which osseous union of the 

 epiphyses to the finger-bones was effected. As the age of 



Other observatio 



Dormouse 



Guinea-pig 



Lop rabbit — 



Buck 



Doe 



Cat 



Cat ^... 



Goat V... 



Fox 



Cattle 



Large dogs 



English thoroughbred 



horse 



Hog 



Hippopotamus 



Lion 



English horse — hunter 



Arab horse 



Camel 



Man 



Elephant 



Elephant 



Ainslie Hollis 



Flourens 



R. O. Edwards, p. m 



,, ,, p. m 



St. G. .Mivart 



J. Jennings 



I'egler 



St. G. Mivart 



Ainslie Mollis 



Dalziel, p. m 



Ainslie Hollis 



James Long 



" Chambers's Encyclop;edia ".. 



St. G. Mivart 



Blaine 



Ainslie Hollis 



Klourens 



Buffon, f. t. 1 



Darwin 



C. F. Holder and Indian 

 hunters 



15-20. 14 Klourens. (Iresswell 



40-50 Grindon 



90-100 I'lourens 



NO. 1534. VOL. 59] 



