Here's why:
* Mammals are a Class: They are a large and diverse group, classified as Class Mammalia.
* Many Genera within Mammals: Within this class, there are many different genera, each containing multiple species. For example, the genus *Canis* includes dogs, wolves, and coyotes.
* No Single Genus Represents All Mammals: It's impossible to have a single genus that encompasses the entire diversity of mammals, from tiny shrews to massive whales.
So, instead of a single genus, mammals are organized into many different genera based on their shared characteristics.