text:  "If then it be so natural to the human mind even in its highest state of culture to be incapable of conceiving and on that ground to believe impossible what is afterwards not only found to be conceivable but proved to be true; what wonder if in cases where the association is still older more confirmed and more familiar and in which nothing even occurs to shake our conviction or even to suggest to us any conception at variance with the association the acquired incapacity should continue and be mistaken for a natural incapacity? It is true our experience of the varieties in nature enables us within certain limits to conceive other varieties analogous to them