"Is it philosophical, is it quite allowable, to assume without evidence from fossil plants that the family or any of the genera was once larger and wide spread? and occupied a continuous area?"

This quote by Asa Gray is questioning the validity of thinking that a family members or genus of plants was when larger and much more prevalent without any evidence from fossil plants. Gray is asking if it is philosophically acceptable to make such a presumption without any proof. He is suggesting that it is not important to make such a case without any evidence to back it up. This quote is a tip that it is important to be cautious when making presumptions and to constantly seek proof to support them. It is also a suggestion that presumptions need to not be taken as fact without any proof. Gray's quote is a suggestion that it is essential to be conscious of the assumptions we make and to constantly search for proof to back them up.