dependent upon plants for their food supply. Plants, therefore, may be looked upon as the primary producers of life, and animals as the consumers. It is in this respect that the horticulturist becomes interested, in that certain of these consumers destroy too many of the plants grown by him for other purposes; fortunately, not all of the consumers are destructive; many are of very great use to the horticulturist and mankind in general.

The last point is well illustrated by the following classification of the animal kingdom based upon the part it plays in human welfare; this is a modification of the scheme adopted by the British Museum of Natural History:—

Group I.—Wild or domesticated animals used by man as beasts of burden, source of food, or in the manufacture of various products—​e.g., sponges, crayfish, bees, silk-worms, shell-fish, and various vertebrates, as fish, birds and mammals.

Group II.—Animals detrimental to man’s welfare, attacking man himself; animals and plants of value to him, or the products derived therefrom—​e.g., Protozoa, parasitic worms, mites, insects, and such vertebrates as certain birds and mammals.

Group III.—Animals aiding man’s welfare, as scavengers, or by pollinating flowers, or by attacking and checking such animals as are included in Group II.—​e.g., Protozoa, parasitic worms, earthworms, parasitic insects, spiders, and such vertebrates as certain birds and mammals.

An analysis of the above classification shows that animals both aid and hinder the progress of man, hence the use of the terms “beneficial” and “destructive.” In nature, however, these terms are not altogether applicable in the same sense, since the balance maintained between animals and plants under natural conditions is an extremely fluctuating one, though sufficient for natural purposes; with man, however, the case is different. In order to compete in the world’s markets, and to supply the growing demands of increasing population, a much higher and dependable standard of productivity is required than is found in nature. Consequently, whilst utilising, and increasing the efficiency of the so-called natural enemies as auxiliaries in his fight against destructive animals, man has found it necessary to develop an effective system of artificial control, involving chemicals, resistant plants, cultivation, crop rotation, etc., for the purpose of maintaining a more stringent balance to meet his requirements.

Historical Review of New Zealand Conditions.

The animal population of European New Zealand is very different from that of pre-European times, a position brought about naturally enough by the changes resulting from agricultural development as practised in the Old World, and the consequent creation of an environment foreign to the country.

Though the official date of the settlement of New Zealand by Europeans is 1840, the influences, inaugurating that upheaval of the natural conditions which was later to have such a marked effect on the economic development of the country, had commenced many years earlier.

Fig. 1.—Some common animals grouped to represent the twelve main divisions of the animal kingdom.

When the first Europeans set foot in New Zealand, they must have been impressed by their unique surroundings, totally different from anything to be met with in the Old World. They found the country dominated by a forest quite unlike the forests of any other land, and inhabited by an animal population presenting many unusual features. This terrestrial population was characterised by an abundance of insects and spiders, and a paucity of vertebrates excepting the birds; the vertebrates consisted of a species or two of frogs, a few species of lizards, some 200 species of birds, and two species of bats, the last being the only terrestrial mammals. In fact, the insects, spiders and birds were the dominant animals, a feature common to other parts of the world, but the scanty vertebrate population, other than birds, was a characteristic of primeval New Zealand.

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.

These comparatively simple compounds from which the plants elaborate their nourishment are the raw food materials, and that they must always be available for plant growth, is evident when one considers the vast areas of vegetation that cover, with the exception of desert regions, the surface of the earth. Under moist climatic conditions it has been calculated that some 500 tons of carbon dioxide and 1,000,000 tons of water, having the raw food materials in solution, are used annually by one square mile of dense forest. For their development, therefore, plants require:—

(1) Sunlight as the source of energy for the carrying on of their life functions;

(2) Air for the