That Mary got to her childbearing period sooner than she would have if she hadn't insisted on marrying John before he was ready to support her. Those two years would have been childless years in any case. But they would probably, if it hadn't been for Mary's money, have been lengthened into four or five.
Of course, later marriages in themselves tend to reduce the number of children. As to quality, however, the evidence is not clear. There is even some reason to think that a moderate postponement is conducive to an improvement in quality.
Did you ever read Havelock Ellis's book called "A Study of
British Genius"?
He made a list of the most distinguished of Eminent British Persons and studied everything about them from their religious opinions to the color of their hair.
In the matter of the age of their parents, he finds that the average age of the father at the birth of the person of genius was thirty-seven years, while the average of the mother was thirty-one. His conclusion is: "On the whole it would appear, so far as the evidence goes, that the fathers of our eminent persons have been predominantly middle-aged and to a marked extent elderly at the time of the distinguished son's birth; while the mothers have been predominantly at the period of greatest vigor and maturity and to a somewhat unusual extent elderly. There has been a notable deficiency of young fathers and, still more notably, of young mothers."
And did you ever see the study which Mr. R. S. Holway made for the Department of Education of Leland Stanford University on "The Age of Parents: Its Effects upon Children"? His conclusions are:
"In most physical qualities the children of mature parents tend to come out best.
"In mental ability the children of young parents show best at an early age but rapidly lose their precocity.
"The elder children who show best tend to be the children of mature and old parents.
"The children of elderly mothers show a tendency to superiority throughout."