Maybe we are, thought the biologist. He said, "The point is that
survival was difficult. Birds could and did fly to more suitable
climates; quite a few of them survived. Only one species of mammals
managed to come through."
"Your facts are not straight," observed Hafner. "There are four
species, ranging in size from a squirrel to a water buffalo."
"One species," Marin repeated doggedly. "They're the same. If the food
supply for the largest animal increases, some of the smaller so-called
species grow up. Conversely, if food becomes scarce in any category,
the next generation, which apparently can be produced almost
instantly, switches to a form which does have an adequate food
supply."
"The mice," Hafner said slowly.
* * * * *
Marin finished the thought for him. "The mice weren't here when we got
here. They were born of the squirrel-size omnivore."