Charles Darwin was influenced by many writers, philosophers, and friends. One of his influences was Thomas Malthus, a late-eighteenth century economist who wrote: "Essay on the Principle of Population" (1798), which Darwin read and was inspired by. The central theme of Malthus' work was that population growth would always overpower food supply growth, creating perpetual states of hunger, disease, and struggle. The natural, ever-present struggle for survival caught the attention of Darwin, and he extended Malthus' principle to the evolutionary scheme. Darwin's finches are a group of about fifteen species of passerine birds. They are often classified as the subfamily Geospizinae or tribe Geospizini. They belong to the tanager family and are not closely related to the true finches. The most closely known relative of the Galápagos finches is T. obscura. They were first collected by Charles Darwin on the Galápagos Islands during the second voyage of the Beagle. The finches of the Galapagos were used by Darwin to understand the natural selection. The results of artificial selection were also helpful to Darwin to synthesise the concept of natural selection. Mendel's laws of inheritance were not helpful to Darwin. Thus, option D is correct.