(a) Palaeontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals that have been replaced by rock material or impressions of organisms preserved in rock. (b) Vestigial organs are the structures in body that are found in reduced condition and serve no function. They, however, resemble and correspond to the same organs which are well developed and useful in other animals or were useful in ancestors of organisms. For example appendix in humans.  (c) The geologic time scale is the “calendar” for events in Earth history. It subdivides all time into named units of abstract time called—in descending order of duration—eons, eras, periods, epochs, and ages.  (d) The ancestral characters which are supposed to have been lost during the course of evolution may sometimes reappear in the organisms suddenly as abnormal characters. This is known as atavism. Large canine teeth in human on rare occasions and rare development of small tails in human babies are examples of atavistic characters. (e) Organs that are similar in their structural plan and in their embryonic developments are called homologous organs. Wings of birds, bat and pterodactyls are homologous because structurally and embryologically they are similar.  (f) It is now evident that during embryonic development all the plants and animals pass through the similar stages, their ancestors went through in the long course of evolution. Thus in brief, it can be said that the embryonic development or ontogeny is recapitulation of the ancestral history or phylogeny or in other words, “ontogeny repeats phylogeny “. This is known as ‘Biogenetic law’. Ontogeny is the development of the individual and the phylogeny is the development of the species or race. The law was propounded by E. Haeckel.