146 REY. W. SERLE ON 
the atmosphere—rainy seasons or spells of frost. I do not 
say that birds could argue these things out, but all through 
Nature there are seen mysterious laws that fit things to 
ends, and issue in what are called fittest arrangements. 
The arrangements we see; it is the process that has cul- 
minated in the arrangement that we wish to show. Of course 
that is not the sole explanation. In fact, I should have 
mentioned first that experience had told of congested areas. 
The mortality among the feathered tribes must have been 
enormous before the habit of migration was formed. We cannot 
trace this as we can that of the extinct oxen of South America, 
for example, whose bones are found huddled together in such 
great abundance in some districts. From time to time we hear 
of it on a small scale, when the locust flights in South 
Africa suddenly fail, and the locust birds that were rearing 
their young in thousands suddenly forsake their young 
altogether to ward off starvation from themselves. The 
memories of these things remain with birds, and in time, 
through influences of heredity, lead to the habit of migration. 
Thirdly, if the failure of the food-supply drove birds 
southwards, it is the lack of suitable food for their young 
that brings them back in the summer season. Even the 
birds that remain in tropical regions all the year round 
lay fewer eggs than birds in temperate parts, and some of 
the larks that remain on the burning plain of Africa lay 
but one egg. The common water-hen of our marshes, which 
with us has a clutch of nine eggs, in more tropical parts is 
content with a clutch of four. Young birds require soft food, 
whether warbler or finch; and insects are abundant in our 
summer season. It is only now and again that our summers 
are so dry that the parent birds are nearly exhausted in 
catering for the wants of their young. 
Fourthly, there is less danger in the rearing of young birds 
in these temperate parts. No monkeys are in our forests to 
pry into every hole and corner for the callow young or tit-bit 
of an egg; no lizards darting about in all directions to find 
the prize in the open clearings where monkeys feared to go; 
no scorpions to enter the holes in African solitudes where the 
rockchats brood in the darkness over their blue spotted eggs ; 
or if art and cunning have concealed the nests from all 
