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.