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Free guide for Reptiles, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. They do not have an aquatic larval stage. Most reptiles are oviparous, although several species are, as were some extinct aquatic clades — the fetus develops within the mother, contained in a rather than an eggshell. As amniotes, eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant range in size from a which can grow up to 17 mm (0.7 in) to the 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).

Sub-disciplines of biology are defined by the scale at which life is studied, the kinds of organisms studied, and the methods used to study them. The discovery of the physical representation of heredity came along with evolutionary principles. In the 1940s and early 1950s, experiments pointed to DNA as the component of chromosomes that held the trait-carrying units that had become known as genes. A focus on new kinds of model organisms has developed.

Free guide that can help anyone who wants to find out more on the subject. Reading is fun and free. A central organizing concept in biology is that life changes and develops through evolution, and that all life-forms known have a common origin. The theory of evolution postulates that all organisms on the Earth, both living and extinct, have descended from a common ancestor or an ancestral gene pool. This universal common ancestor of all organisms is believed to have appeared about 3.5 billion years ago. Feel free rate us or leave your replies.

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