PROBLEM OF MIGRATION. 447 
marshes, and therefore more or less closed to birds adapted to 
woods, thickets, or dry fields. They are, however, suited to the 
needs of waders, and these are the dominant summer birds. 
But later in the year, when emigration takes place, the structures 
that fitted the waders for a life in the Arctic marshes prevent 
them spending the winter in places other than those bearing at 
least a superficial resemblance to the summer home. For the 
rest of the year they become “‘shore birds”; and we must look 
upon the generalized and most advanced form of migrant as 
birds passing the summer near the Pole, and with a winter 
range that is nothing more than the slender tracery of the 
littoral zones of the remainder of the entire globe. 
With ‘‘land”’ birds matters are rather different, and the 
generalizations are unavoidably broad. So many migrants are 
insectivores that we may well take all these as forming a type 
species. In England, in winter, food is lacking. In Africa, on 
the contrary, the ‘‘ wet season” has been accompanied by an 
increase in insects, and the birds pass south to utilize this food; 
and, incidentally, prevent undue increase on the part of the 
plant pests, with the possible destruction of the vegetation. 
With the return of summer England becomes again a fit manu- 
factory of protoplasm. The trees and plants break into leaf. 
Insects appear and increase, and threaten the very existence 
of the plants; and, at the most critical season, a vast tide 
of birds flows north to devour the insects and so preserye the 
vegetation. 
Here, in winter, the poverty of solar energy means a dearth 
_of foliage, and a consequent lack of food for the insects. These 
animals come to a standstill during the cold season. In the 
Tropics the conditions are not quite the same. It must be 
remembered that water is as essential as sunlight to the growing 
plant, and a dry desert is thus as barren as an Arctic winter 
continent. But in many parts of the Tropics the year is divided 
into two seasons, a wet andadry. ‘The former corresponds to 
the northern summer in being a period of exuberant growth. 
This is the breeding time of many birds and other animals, 
and we know that beyond the needs of the sedentary organisms, 
there is a surplus sufficient to support the migrants that have 
passed south to avoid the northern winter. In those parts of 
