139. Read the following passage, then answer the question below.

In 1912, the German meteorologist Alfred Wegener proposed that the continents had once formed a single landmass and had since drifted apart. He pointed out that the coastlines of South America and Africa fit together like pieces of a puzzle, and that identical fossils of land animals and plants appear on continents now separated by oceans. Most geologists rejected the idea, not because the evidence was weak, but because Wegener could not explain what force could push continents through solid ocean floor. Critics noted that his proposed mechanisms, including the pull of the tides, were far too feeble.

The missing mechanism emerged after the Second World War, when mapping of the seafloor revealed a continuous mountain chain winding through the oceans. In the early 1960s, Harry Hess suggested that this ridge marks a place where molten rock rises from below, forming new ocean crust that spreads away on either side, while old crust is destroyed elsewhere. Supporting evidence came from the magnetism of seafloor rocks: iron-bearing minerals record the direction of Earth's magnetic field when they cool, and since the field has reversed many times, rocks on either side of a ridge show matching stripes of normal and reversed polarity.

By the late 1960s, these findings were combined into the theory of plate tectonics, in which Earth's rigid outer shell is divided into plates that move a few centimeters a year, about as fast as fingernails grow. Plates collide, pull apart, or slide past one another, and most earthquakes and volcanoes occur along these boundaries. Wegener, who died in Greenland in 1930, did not live to see his central idea vindicated, though the way it came to be accepted differed from what he envisioned: continents do not plow through the ocean floor but ride along with it.

Why does the author compare plate motion to the growth of fingernails?