Using various resources such as your school library or the internet and discussions with your teacher, trace the evolutionary stages of any one animal say horse.

The evolution of horse started with Eohippus during Eocene period. It involved the following evolutionary stages.

Gradual increase in body size

Elongation of head and neck region

Increase in the length of limbs and feet

Gradual reduction of lateral digits

Enlargement of third functional toe

Strengthening of the back

Development of brain and sensory organs

Increase in the complexity of teeth for feeding on grass

The evolution of horse is represented as  (figure)

$(i)$ Eohippus

It had a short head and neck. It had four functional toes and a splint of $1$ and $5$ on each hind limb and a splint of $1$ and $3$ in each forelimb. The molars were short crowned that were adapted for grinding the plant diet.

$(ii)$ Mesohippus

It was slightly taller than Eohippus. It had three toes in each foot.

$(iii)$ Merychippus

It had the size of approximately $100\, \mathrm{cm} .$ Although it still had three toes in each foot, but it could run on one toe. The side toe did not touch the ground. The molars were adapted for chewing the grass.

$(iv)$ Pliohippus

It resembled the modern horse and was around $108\, \mathrm{cm}$ tall. It had a single functional toe with splint of $2^{\text {nd }}$ and $4^{\text {th }}$ in each limb.

$(v)$ Equus

Pliohippus gave rise to Equus or the modern horse with one toe in each foot. They have incisors for cutting grass and molars for grinding food.