One or other of these causes will explain almost every case of migration. Birds are lively and intellectual, imaginative and affectionate creatures, and all their movements are not dictated by mere necessity. They love the hedge and bush where they were born, they return to the same tree, or the same spot under the eave. On the other hand, they like to roam about the fields and woods, and some of them travel long distances during the day. When the pleasurable cares of the nest are concluded, it is possible that they may in some cases cross the sea solely for the solace of change. Variety of food is itself a great pleasure. By prehistoric memory is meant the unconscious influence of ancient habit impressed upon the race in times when the conformation of land and sea and the conditions of life were different. No space is left for a mysterious agency; migration is purely natural, and acts for the general preservation. Try to put yourself in a bird's place, and you will see that migration is very natural indeed. If at some future period of the world's history men should acquire the art of flying, there can be no doubt that migration would become the custom, and whole nations would change their localities. Man has, indeed, been always a migratory animal. History is little beyond the record of migrations, how one race moved on and overcame the race in front of it. In ancient days lots were cast as to who should migrate, and those chosen by this conscription left their homes that the rest remaining might have room and food. Checking the attempted migration of the Helvetii was the beginning of Caesar's exploits. What men do only at intervals birds do frequently, having greater freedom of movement.