Instant messenger and railway semaphore: what they have in common

1. Step 1. Instant messenger — evolved from — SMS. Messengers displaced SMS.
2. Step 2. SMS — works like — Electrical telegraph. A short text message is the heir of the telegram.
3. Step 3. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.
4. Step 4. Optical telegraph — evolved into — Railway semaphore. The semaphore idea.

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