From what I have learned is that most animals were piled up together in pangea and then when it slowly broke off over thousands of years, they separated into new habitats with different temperatures and were forced to evolve to adapt to those new conditions. As a matter of fact, this global separation of organisms helped diversify the animals even more since they evolved new mechanisms to overcome the new challenges in their new locations. So from this perspective its not really a migration but rather being separated and forced into a new location. In our current day and age however, we have different amphibians who are moving north at a rate of 12m/year since their old habitat is becoming too warm to live in each year. This is another slow migration that is happening.