The process by which that ancient chapter of history was recovered is very simple. Take the words which occur in the same form and with the same meaning in all the seven branches of the Aryan family, and you have in them the most genuine and trustworthy records in which to read the thoughts of our true ancestors, before they had become Hindus, or Persians, or Greeks, or Romans, or Celts, or Teutons, or Slaves. Of course, some of these ancient charters may have been lost in one or other of these seven branches of the Aryan family, but even then, if they are found in six, or five, or four, or three, or even two only of its original branches, the probability remains, unless we can prove a later historical contact between these languages, that these words existed before the great _Aryan Separation_. If we find _agni_, meaning fire, in Sanskrit, and _ignis_, meaning fire, in Latin, we may safely conclude that _fire_ was known to the undivided Aryans, even if no trace of the same name of fire occurred anywhere else. And why? Because there is no indication that Latin remained longer united with Sanskrit than any of the other Aryan languages, or that Latin could have borrowed such a word from Sanskrit, after these two languages had once become distinct. We have, however, the Lithuanian _ugnìs_, and the Scottish _ingle_, to show that the Slavonic and possibly the Teutonic languages also, knew the same word for fire, though they replaced it in time by other words. Words, like all other things, will die, and why they should live on in one soil and wither away and perish in another, is not always easy to say. What has become of _ignis_, for instance, in all the Romance languages? It has withered away and perished, probably because, after losing its final unaccentuated syllable, it became awkward to pronounce; and another word, _focus_, which in Latin meant fireplace, hearth, altar, has taken its place.
Suppose we wanted to know whether the ancient Aryans before their separation knew the mouse: we should only have to consult the principal Aryan dictionaries, and we should find in Sanskrit _mûsh_, in Greek [Greek: mus], in Latin _mus_, in Old Slavonic myse, in Old High German _mûs_, enabling us to say that, at a time so distant from us that we feel inclined to measure it by Indian rather than by our own chronology, the mouse was known, that is, was named, was conceived and recognized as a species of its own, not to be confounded with any other vermin.