Cope's rule states that evolution tends to increase body size over geological time in a lineage of populations.
Evolutionary trends towards an increase in body size are common in the fossil record. For example, the Eocene ancestors of modern horses were about the size of a dog. Since then, in the lineages showing the largest increases, horses have evolved to become as much as 10 times heavier.
Cope's rule is named after the paleontologist Edward Drinker Cope.
Figure: over the last sixty million years, the average weight of horses has increased ten times.