In recent years, however, more public attention has been given to the lower animals owing to the detrimental influence of many upon agricultural development as well as upon public health. That such animals are capable of ranking as fundamental factors hindering human progress, may be realised when it is considered that, of the invertebrates, insects alone comprise nearly four-fifths of the whole animal kingdom! This has been graphically illustrated as follows by F. E. Lutz, of the American Museum of Natural History:--Extend the arms and fingers at right angles to the body, and let the distance from the tip of the middle finger of one hand to that of the other represent the number of different kinds of living animals; then the last joint of the middle finger of the right hand will be proportionate to the number of mammals (kangaroos, hoofed animals, rabbits, man, etc.), the second joint to the reptiles and their relations, the first joint to the birds, and the distance between the knuckles and the wrist to the fishes. “In other words, you can hold the so-called zoological gardens and their aquarium annexes in one hand.” Finally, the distance between the wrist of the right arm and the tip of the middle finger of the left will proportionately represent all the known species of invertebrates, and of this section of the extended arms all except between a wrist and an elbow will be insects. The zoologist classifies the animals under twelve main divisions, of which eleven contain the invertebrates and one the vertebrates; these divisions are arranged in a series, the first containing the simplest or lower animals, and the last the most complex or highest. A glance at this classification will serve to give some idea of the relative position in the animal kingdom of the animals which will be dealt with in the following pages. The very lowest forms, belonging to the first division, are micro-organisms known as the Protozoa; they inhabit water and soil, and live upon their own kind or upon minute plants, including bacteria, or are parasitic upon the higher animals, some of these parasites causing such diseases as malaria. The Protozoa are single units of living matter (protoplasm), and may be referred to as the one-celled animals; they are mostly microscopic, and lead an independent life, or are associated in colonies, but are capable, as a rule, of carrying on independently all the functions of life, though there are no organs such as those of digestion, respiration, and circulation, as we know them in the higher animals. It is amongst such simple forms that the distinction between the lowest animals and plants ceases to be clear. As will be discussed later, there is evidence that certain Protozoa have an important influence on soil fertility.