Perry’s intuition turned out to be visionary. He continued to maintain that the failure of Kelvin’s model to produce greater ages was a direct consequence of Kelvin’s assumption of a homogeneously conductive Earth, and that this limitation could be overcome if one allowed the Earth’s mantle to convect. It took the geologists of the twentieth century a few decades to prove Perry right. The realization that convection was possible, even within what appeared to be a rather solid mantle, played an important role in the eventual acceptance of the idea (first introduced in 1912 by the German scientist Alfred Wegener) of plate tectonics and continental drift. Not only can heat be transported by fluidlike motion but also entire continents can move horizontally over long periods of time. The precise conditions at the interface between the Earth’s inner core and the outer part continues to be a hot topic (no pun intended) of research even today.

Perry concluded his last article on the subject of the age of the Earth with an unambiguous statement:

From the three physical arguments [tidal retardation of the Earth’s spin; the cooling of the Earth; and the age of the Sun], Lord Kelvin’s higher limits are 1,000, 400, and 500 million years. I have shown that we have reasons for believing that the age, from all three, may be very considerably under estimated. It is to be observed that if we exclude everything but the arguments from mere physics, the probable age of life on the earth is much less than any of the above estimates; but if the palaeontologists have good reasons for demanding much greater times, I see nothing from the physicist’s point of view which denies them four times the greatest of these estimates.

Perry saw nothing wrong with 4 billion years for the Earth’s age, fairly close to today’s determination of about 4.5 billion years.

Perry’s work created the first crack in Kelvin’s seemingly unshakable calculations, by challenging the postulates that Kelvin made concerning the Earth’s solidity and homogeneity. There was, however, another crucial hypothesis in Kelvin’s estimate of the age of the Earth: that there were no unknown internal or external energy sources that could compensate for the heat losses. Events toward the end of the nineteenth century demolished this premise too.