(I) Climatic conditions vary widely from place to place and from season to season, but there is a recognisable order and pattern in this great variety of climates. (II) The tropical climate, with its wet and dry seasons, contributes to the richness of the West Indies natural environment. (III) South American tropical plants of every sort are found, but each island also has its own varieties of plants, insects, birds and reptiles.(IV) European colonisation and the immigration of peoples from all over the world have also brought many other species of plants and animals to the islands. (V) Thus, native and introduced species now intermingle in their natural and human-modified ecological systems.