For although one can give some explanation of the formation of the fetus in general, as Descartes has tried successfully enough, nevertheless it is very difficult, without this communication of the mothers brain with the child's, to explain why a mare does not give birth to a calf, or a chicken lay an egg containing a partridge or some bird of a new species; and I believe that those who have mediated on the formation of the fetus will be of this opinion.