448 THE ZOOLOGIST. 
the Tropics that are not so much under the influence of seasonal 
rains the year must be (as a logical outcome of our knowledge 
of biology) divided into a period of insect abundance and im- 
poverished plants, and another of freedom from insects and large 
opportunities of storing potential energy. The first would 
thus correspond to the Arctic summer and the wet season of 
the Tropics. 
Leaving out of account the conditions obtaining in the sea 
(and possibly aérial migration is exceeded by the passage of 
marine animals), and looking upon the soil of the entire globe 
as being equally fitted to support plants, we shall find that the 
areas of what may be termed the greatest biogenetic intensity 
are precisely those best provided with water and with sunlight. 
Over these areas plants are most abundant, and here we expect 
to see the greatest bulk of animal life. But these very biological 
optima are constantly changing, owing to the disappearance or 
shrinking of one or both of the two essentials, light and water : 
the freezing of water, of course, removes it at once from the 
service of plants. The vegetation, being primarily a stationary 
form of life, ceases to collect and fix the solar energy, and comes 
to rest after storing a sufficient reservoir for the following 
season in the shape of seeds or in the form of starch. 
The animal life has two alternatives, hybernation, or migra- 
tion to the opposite biogenetic pole; and, as we know, migration 
is the course usually adopted. The general trend of the move- 
ment is from north to south, in the path of the summer; but 
there is no theoretical reason why the course of migration should 
not take the form of a series of radii in all directions from a 
circumscribed area rendered biologically non-supporting by the 
disappearance of water. It is, however, more than doubtful if 
these local migrations could have any real connection with the 
vast currents of the typical migrants that sweep to and fro each 
spring and autumn with all the precision and much of the 
magnificence of one of the heavenly bodies. 
Seebohm noticed that birds migrated in spring to the lightest 
parts of their ranges, and he was thus led to suggest that 
the longer northern summer day, as contrasted with the twelve 
hours of the tropical day, gave the migrants more time in 
which to seek food and rear their young. Other writers have 
