Page:Popular Science Monthly Volume 67.djvu/62

56 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 "Duration 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 individuals of a species obtain some advantage in the struggle for existence 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.