New Zealand being a country fitted for agriculture, settlement by Europeans naturally resulted in extensive and rapid changes, since the settlers brought with them the knowledge, implements, animals and plants of the civilised world; and to make way for settlement, it was necessary to remove the forests and drain the swamps, and to replace them with cultivated crops and pastures. These activities have been so thorough, that, within a period of some 90 years practically the whole of the original North Island forests, and the greater part of those of the South Island, have been cleared. An outstanding feature of these changes is that many of the pests associated with the agricultural animals and plants have been brought to New Zealand with the animals and plants they infest, and these exotic pests comprise by far the greater proportion of the destructive animal population, there being but few native species forming the balance. For example, 71 per cent. of the destructive insects are exotic, and 29 per cent. native, while all the parasitic worms of economic importance, all the destructive birds (_e.g._, sparrows) and mammals (_e.g._, deer, wild pigs, and goats) are introduced. The exotic factors that have set up this new environment may be summarised as follows:-- (1) Clearing of the native vegetation. (2) Introduced plants: _e.g._, grasses, forage crops, trees, etc. (3) Introduced game animals: _e.g._, deer, pigs, rabbits, birds, etc. (4) Introduced destructive animals, infesting animals and plants of economic value: _e.g._, parasitic worms, insects, etc. (5) Animals imported to control pests, but which have become destructive themselves: _e.g._, weasels, birds. CHAPTER II. Soil Organisms and Soil Fertility. In the first chapter the plants were referred to as the primary producers of life, and the animals as the consumers; the former not only furnish nourishment for their own growth, but also for the support of the animal world as a whole. Living plants (in reference to green plants) utilise the sun’s energy in the manufacture of their complex food materials from comparatively simple chemical compounds. These latter compounds are carbon dioxide, derived from the air through the agency of leaves, and a weak solution of various chemical compounds in water, derived by means of the roots from the soil, and carried up through the plant to the leaves, where the elaboration into the complex compounds to be utilised by the plants as food takes place.