Answer:
Straight-line evolution
Explanation:
The Horse has undergone continuous evolution from about 55 million years to the recent providing a clear picture of how evolution has taken place and what are the characteristic changes that are being made during its period of evolution.
It has been observed that the horse initially was a dog-like animal and in due course of time, it changed to the first appearance of horse species known as "Eohippus" about 50-40 million years back. It again changed to "Mesohippus" by about 40-30 million years back, followed by "Hipparion" in the last 25 to 3 million years and forming the recent horse.
This gradual change in the structure and appearance of the organism in due course of time is considered to be the straight-line evolution.
