An Introduction to Information Theory: Symbols, Signals and Noise, by John R. Pierce, says the following:
The difficulty which Morse encountered with his underground wire remained an important problem. Different circuits which conduct a steady electric current equally well are not necessarily equally suited to electrical communication. If one sends dots and dashes too fast over an underground or undersea circuit, they are run together at the receiving end. As indicated in Figure II-1, when we send a short burst of current which turns abruptly on and off, we receive at the far end of the circuit a longer, smoothed-out rise and fall of current. This longer flow of current may overlap the current of another symbol sent, for instance, as an absence of current. Thus, as shown in Figure II-2, when a clear and distinct signal is transmitted it may be received as a vaguely wandering rise and fall of current which is difficult to interpret.
Of course, if we make our dots, spaces, and dashes long enough, the current at the far end will follow the current at the sending end better, but this slows the rate of transmission. It is clear that there is somehow associated with a given transmission circuit a limiting speed of transmission for dots and spaces. For submarine cables this speed is so slow as to trouble telegraphers; for wires on poles it is so fast as not to bother telegraphers. Early telegraphists were aware of this limitation, and it, too, lies at the heart of communication theory.
As someone who does not have an electrical engineering background, I find the described phenomenon perplexing. If one sends short bursts of electric current that abruptly turn on and off, why is it true that, depending on the type of circuit, the receiver may receive a smoothed-out current, rather than the discrete currents that were sent? One would expect, naively perhaps, that the signals received would be identical to the signals sent?
I would greatly appreciate it if people could please take the time to answer this using language that can be understood by someone without an electrical engineering background.