56 POPULAR SCIENCE MONTHLY. 



becomes longer. This statement is obviously of no value, for a sea 

 urchin that may grow up in a year, produces millions of eggs, while 

 an elephant that takes thirty years to mature produces only about six 

 young in the whole course of its life. Equally valueless, it appears 

 to me, is Weismann's statement ' that nature does not tend to secure 

 the longest possible life to the adult individual, but, on the contrary, 

 tends to shorten the period of reproduction as far as possible, and with 

 this the duration of life.' 



Coming now to the main question as to how natural selection may 

 be supposed to increase the length of life, Weismann states, that " Dura- 

 tion of life like every other characteristic of the organism is subject 

 to individual fluctuation. From our experience with the human species 

 we know that long life is hereditary. As soon as the long lived in- 

 dividuals of a species obtain some advantage in the struggle for exist- 

 ence they will probably become dominant, and those with the shortest 

 lives will be exterminated." 



Without attempting to offer an elaborate refutation of Weismann's 

 view, I should like briefly to present the following considerations : 



1. It is, of course, almost self-evident that the existence of a 

 species is closely bound up with its powers of reproduction, but it does 

 not follow from this that the length of life has been adjusted to come 

 to an end when the animal can no longer reproduce itself, because it 

 can not reproduce itself any longer. 



2. On the contrary it is more probable that the same causes that 

 have led to the cessation of the powers of reproduction may be closely 

 associated with those that bring about a decline in the general vitality ; 

 so that while death may follow at a variable period after the power of 

 reproduction is lost, the two processes have not been adjusted to each 

 other by some external need, but are part of the same physiological 

 decline. 



3. In the higher animals especially, there may be thousands of 

 immature eggs when the animal ceases to reproduce, as in the case of 

 the human species. It would seem to be greatly to the advantage of 

 a species to have the individual that has surmounted the dangers of 

 youth bring all of its eggs to maturity before it dies; yet such is not 

 the case. The eggs fail to mature not because it is to the advantage 

 or disadvantage of the species to perish after it has set free a part of 

 its eggs, but because the general decline of the organism brings to an 

 end the power to ripen its eggs. 



4. The natural duration of life of each species determines when its 

 reproductive powers begin to decline, and the relation is, therefore, 

 exactly the reverse of that which Weismann assumes ; for the cessation 

 of the reproductive power is determined by the decline of vitality and 

 this decline is not regulated by the cessation of the power to reproduce. 



