James Clerk Maxwell was one of the greatest physicists of the 19th century after whom the famous Maxwell's Demon was named. Today, I found one of his textbooks, Theory of heat, which was first published in 1870.
The great value of such a book, rather old as far as the current level of our knowledge is concerned, is that it opens up a portal to the mind of a great scientist and gives us clues as to how he may have developed some of his ideas.
Near the end of the book, there is an intriguing passage where Maxwell is comparing molecules, still a relatively new concept back then, to species and evolution, also a relatively new idea for that period.
The molecules of the same substance are all exactly alike, but different from those of other substances. There is not a regular gradation in the mass of molecules from that of hydrogen, which is the least of those known to us, to that of bismuth; but they all fall into a limited number of classes or species, the individuals of each species being exactly similar to each other, and no intermediate links are found to connect one species with another by a uniform gradation.Maxwell, it seems, was familiar with at least the basic notions of Darwin's theory of evolution.
We are here reminded of certain speculations concerning the relations between the species of living things. We find that in these also the individuals are naturally grouped into species, and that intermediate links between the species are wanting. But in each species variations occur, and there is a perpetual generation and destruction of the individuals of which the species consist.
Hence it is possible to frame a theory to account for the present state of things by means of generation, variation, and discriminative destruction.
In the case of the molecules, however, each individual is permanent; there is no generation or destruction, and no variation, or rather no difference, between the individuals of each species.
Hence the kind of speculation with which we have become so familiar under the name of theories of evolution is quite inapplicable to the case of molecules.