MAN’S AGRICULTURAL NEEDS 29 
it is to be remembered that the land-areas have to 
provide for all of these. 
The rate of increase of horses, cattle, sheep and pigs 
combined, from 1911 to 1921, was about 0-923 per 
cent. per annum, but from 1913 to 1925 was only 0-355 
per cent. per annum. In countries subject to droughts, 
the numbers of cattle and sheep are found to vary 
enormously. 
Regarding provision for animals and their food, it 
may be noted that there is a certain scheme of exchange 
in Nature in respect of the vitamines produced by 
animals and existing in vegetables. It is not simply 
their energy-values that need to be taken account of 
when foods are considered since their vitamine con- 
stituents are important. For this reason animals will 
always be required. The existing state of things in this 
respect, however, is by no means the best possible. It 
may be noted also that, whatever advance may be 
made by substituting mechanical energy for animal 
power, a considerable number of animals will be 
required always; and this fact ought not to be over- 
looked. 
From the point of view of food-supply, it may be 
observed that sea-mammals, fish, and sea-products 
generally, will doubtless be drawn upon in future to a 
very much greater extent than in the past. They will 
be used for general purposes as well. The possibilities 
of progress in these directions may be considerable, and 
possibly can fairly well be gauged from existing human 
experience. Life in the sea-world is already held in 
check by factors operating within its own domain. 
The multiplying powers of fish are so enormous that, 
but for their consuming one another, the space they 
occupy would become inadequate in a few decades. 
Industrial uses may be found for certain predatory 
fish; in this way no doubt a larger use may be made of 
food-fishes than is now possible. That the possibilities