One of the main arguments against plate tectonics in its early days was the lack of a credible mechanism to explain how continents could move across the Earth's surface.

Here's why:

* No known force strong enough: At the time, scientists couldn't identify a force powerful enough to move massive continental landmasses across the globe. The prevailing idea was that the Earth's interior was essentially solid, with no way for continents to slide around.

* Absence of evidence: There wasn't strong enough evidence of the mechanism of plate movement. The concept of seafloor spreading, which would later provide the key, hadn't been fully developed or widely accepted.

This lack of a plausible driving force led many scientists to reject continental drift and, later, plate tectonics.

It wasn't until the 1960s that the theory of seafloor spreading gained traction, providing a plausible mechanism for continental drift. The discovery of mid-ocean ridges, magnetic striping patterns on the seafloor, and the process of subduction helped solidify the evidence for plate tectonics.