Cretaceous

The Cretaceous Period appears as a deep-time chapter in the history of species movement: seas divided continents, isolation fostered new forms of life, and later reconnection opened routes into new territories. Set against the book’s accounts of modern land management and pesticide use, this history makes ecological change legible on both geological and human timescales.

Geological Isolation and the Making of Species

About a hundred million years ago, flooding seas cut many land bridges between continents. Charles Elton describes the separated regions as “colossal separate nature reserves”: organisms confined there were isolated from others of their kind, and over time developed into many new species. The image of a reserve here is not a managed preserve but a consequence of geography, with separation creating conditions for evolutionary differentiation.

Reconnection and Migration

About 15 million years ago, some land masses joined again. Species then began to move into new territories, renewing the worldwide migration of organisms that the book presents as part of geological and human history. The sequence matters: isolation first allowed distinct forms to emerge; later connections enabled their dispersal. Migration is thus not simply a story of organisms spreading, but one shaped by changing physical conditions and the ecological histories those conditions make possible.

Deep Time and Human Intervention

The period’s account of separation and reconnection establishes a long historical scale for the book’s later treatment of landscape change. In the western plains, sage and animal life evolved in relation to the demands of the land; modern agencies, seeking more grassland for grazing, set out to remove sage. At Bridger National Forest, spraying intended to eliminate sagebrush also killed willows, altering habitat on which beavers depended. These episodes differ profoundly from geological change in pace and cause, but their juxtaposition underscores that living communities are formed through relationships among organisms and their surroundings—and can be disrupted when those relationships are treated as expendable.