This is from p151, in the section in Syntax on pronoun agreement :
We hardly consider little children as persons, because that term gives us the idea of reason and reflection: and therefore the application of the personal relative who, in this case, seems to be harsh: "A child who." It is still more improperly applied to animals: "A lake frequented by that fowl whom nature has taught to dip the wing in water."
This is purely from the point of view of grammar, but it suggests that it is based on the idea that gender is something that is grown into, or acquired with the ability to reason and reflect. Or that adults would take no notice of gender until the powers of reason and reflection were also noticeable. Is it proposing that gender-acquisition is dependent upon the ability to reason and reflect? This is curious given that it was within a world where the statuses of male and female were profoundly different. All rather perplexing, especially given Murray's philanthropic mindset.