Evolution

X, Y and Z are three animals. The animal X can fly, but animal Y can only run on ground or walls. The forelimbs of animals X and Y have the same basic design, but they are used for different purposes, such as flying and running respectively. The animal Z became extinct a long times ago. The study of fossil of Z tells us that it had some features like those of X and some like those of Y. In fact, Z is said to form a connecting link in the evolutionary chain of X and Y.
A group of red beetles lives on green leaves of a tree. Beetles multiply through sexual reproduction. One day, some green beetles are seen among the red beetles. Green beetles breed to produce green progeny. Crows on the tree eat beetles.
Brachiopods are the animals belonging to phylum mollusca but are of rare existence. They show resemblance to clams and live in very cold waters and in deep ocean. Plaeontologists find great interest in Brachiopods as they are useful in providing information related to time sequence and past environmental conditions. The given chart represent the geological time scale for different genera of Brachiopods.
Evolutionary development of a species can be best studied by
At the time when earth originated there was large amount of methane and other hydrocarbons but now in the present atmosphere, it has been replaced by