The poem
Lambs that learn to walk in snow When their bleating clouds the air Meet a vast unwelcome, know Nothing but a sunless glare. Newly stumbling to and fro All they find, outside the fold, Is a wretched width of cold.
As they wait beside the ewe, Her fleeces wetly caked, there lies Hidden round them, waiting too, Earth's immeasureable surprise. They could not grasp it if they knew, What so soon will wake and grow Utterly unlike the snow.
This line; ‘Earth’s immeasurable surprise’ illustrates Larkin’s view that life never pans out as you might expect and that it is amazing and unpredictable. It is more obviously positive than many of his other works and seems to me to express a joy for the world without there needing to be any greater meaning such as a presence of a god or faith of some kind. It is praising the harshness and tenderness of nature, and life in its simplest form.
Why do people often feel the need to find a "message" in works of literature? Rarely ever is it appropriate to read a far-reaching allegory of human nature in these things. The poem is fourteen lines. It's a gentle, subtle thing, and part of the beauty is in that subtlety!