What is the best explanation of that determination?
All of Trease's children would be measuring lives.
The rule of convenience closes the class of beneficiaries when any grandchild reaches the age of 21.
There is a presumption that Trease intended to include only those grandchildren born prior to his death.
There is a subsidiary rule of construction that dispositive instruments are to be interpreted so as to uphold interests rather than to invalidate them under the Rule Against Perpetuities