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354
Hereditary Genius
It will be found that an increase of 1.5 in each generation,
accumulating on the principle of compound interest during 3.75
generations, becomes rather more than 18/4 times the original
amount; while an increase of l.25 for 2.5 generations is barely as
much as 7/4 times the original amount. Consequently the increase of
the race of M at the end of a century, will be greater than that of N
in the ratio of 18 to 7; that is to say, it will be rather more than 2.5
times as great. In two centuries the progeny of M will be more than 6
times, and in three centuries more than 15 times, as numerous as
those of N.
The proportion which the progeny of M will bear at any time, to the
total living population, will be still greater than this, owing to the
number of generations of M who are alive at the same time, being
greater than those of N. The reader will not find any difficulty in
estimating the effect of these conditions, if he begins by ignoring
children and all others below the age of 22, and also by supposing the
population to be stationary in its number, in consecutive generations.
We have agreed in the case of M to allow 3.75 generations to one
century, which gives about 27 years to each generation; then, when
one of this race is 22 years old, his father will (on the average of
many cases) be 27 years older, or 49; and as the father lives to 55, he
will survive the advent of his son to manhood for the space of 6
years. Consequently, during the 27 years intervening between each
two generations, there will be found one mature life for the whole
period and one other mature life during a period of 6 years, which
gives for the total mature life of the race M, a number which may be
expressed by the fraction (6+ 27) /27, or 33/27. The diagram
represents the course of three consecutive generations of race M: the
middle line refers to that of the individual about whom I have just
been speaking, the upper one to that of his father, and the lower to his
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