Variations in organisms

Darwin brought back from his scientific travels the conception that plant and animal species are not constant but subject to variation. In order to make further researches along these lines after his return home there was no better field available than that of breeding of animals and plants. Darwin found that this breeding produced artificially, among animals and plants of the same species, differences greater than those found in what are generally recognized as different species. Thus was established, on the one hand, the variability of species, and on the other, the possibility of a common ancestry for organisms with different specific characteristics.

Darwin then investigated whether there were not possibly causes in Nature which would in the long run¹ produce in living organisms changes similar to those produced by artificial breeding. He discovered these causes in the disproportion between the immense number of germs² created by Nature and the insignificant number of organisms which actually attain maturity. But as each germ strives to develop, there necessarily arises a struggle for existence. And it is evident that in this struggle those individual organisms which have some particular characteristic, however insignificant, which gives them an advantage in the struggle for existence will have the best prospect of reaching maturity and propagating themselves. Those individual organisms which do not possess these characteristics succumb more easily in the struggle for existence and gradually disappear. In this way a species is established through natural selection, through the survival of the fittest.

Notes and Commentary

¹ – in the long run – з часом

² – germ – зародок, ембріон; зав’язь

Soil: ecological aspect

Soil is a mixture of humus, decayed organic matter, and particles of weathered rock, sand, silt, and clay. Clay is perhaps the most important component of soil because it helps hold water in the soil and containes many minerals required by plants for growth. The types of minerals found in clays are largerly dependent on the climate. Humus is equally important because it helps separate clay and sand partcles, allowing more water and air to enter the soil. It also provides food for soil organisms, as well as minerals for plants.

The kind of soil (determined by varying mineral contents, nutrients, and amounts of water) helps determine the organisms that live there. Roses and asparagus grow in different types of soil. Prairie dogs and earthworms live in different types of soil. Specialized grasses and ground covers grow best in specific soil types.

Some herd animals such as cattle change the physical environment when they overgraze. Overgrazing results in the destruction of plant life, which in turn changes the soil. Without plants, soil will erode and the ecosystem will change. Humans are responsible for changes in the ecosystem.

Prospecting with plants

One day in the summer of 1959, geologist Helen Cannon was returning home after a day's field work on the Colorado Plateau, USA. Stopping to rest a moment, she let her horse to eat some grass growing along the road. Shortly afterward the animal died. Mrs. Cannon collected some of the grass that caused her horse die and asked a chemist to analyse it. It was found to be rich in selenium, a highly poisonous metallic element. Mrs. Cannon learned why her horse died. But she learned also a more interesting fact, for she knew that selenium usually lies together with uranium. And so a valuable deposit of uranium was found near the place where the grass had grown.

This incident shows that mineral deposits can be found by using plants.

The use of plants in looking for minerals is called botanical prospecting. It can be done by one of three methods. First, analyse the chemical composition of plants to find the minerals. Second, map the places of growth of particular species of plants – indicator plants – that grow only in soil that is rich in this or that mineral. And third, note changes that are made by certain soil minerals in the size or form of plants.

Why not just analyse the soil, then? This method is not always good. One sample of soil may differ fully of the soil over a wide area. A plant, on the other hand, sends roots down into the soil. It absorbs minerals from a large part of ground together with water which contains minerals in solution¹. In this way minerals become concentrated in the plant.

The first who used botanical prospecting was the Russian scientist S.M. Tkalitch in 1938. He used the first and the second methods in looking for deposits of iron-containing minerals in eastern Siberia. He found that grasses growing above iron deposits contain iron.

Today it is common to have a geobotanist, a man trained in both geology and botany, the science of plants, in every geological expedition. Botanical prospecting has been used here in looking for deposits of boron, nickel, cobalt, iron, chromium and molybdenum.

The use of indicator plants is the simplest botanical prospecting method. No chemical analysis is necessary. Nor a detailed knowledge is needed of how various minerals affect the size or form of plants. Where indicator plants are present, a prospector needs only make a map of their distribution to find possible mineral-rich areas.

Since the time when Helen Cannon's horse died from eating selenium containing grass the United States govern ment, too, has begun botanical prospecting studies. Scientists are beginning to understand that this technique can open up to prospectors large areas of the world now hidden² in forests or tropical jungles.

Notes and Commentary

¹ solution - розчин

² hidden - прихований