(4)

Natural selection is the most widely accepted theory concerning the principal causal mechanism of evolutionary change propounded by Charles Darwin and Alfred Russel Wallace. It results from the differential reproduction (some members of a population produce abundant offspring, some only a few and still others none), one phenotype as compared with other phenotypes in the same population. This determines the relative share of different genotypes which individuals possess and propagate in a population. Industrial melanism supports evolution by natural selection. It is an adaptation where the moths living in the industrial areas developed melanin pigments to match their bodies to the tree trunks.

(3)

The organs which have the same fundamental structure but are different in functions are called homologous organs. These organs follow the same basic plan of organisation during their development. But in the adult condition, these organs are modified to perform different functions as an adaptation to different environments. The forelimbs of cat, lizard, whale and bat have the same basic structural plan. In each case the forelimb consists of humerus, radio-ulna, carpals, metacarpals and digits. The skeletal parts of the forelimbs of all these vertebrates are similar in structure and arrangement. But the forelimbs of these animals have different shapes and functions. In lizard they are used for walking, in cat for running, in whale for swimming and in bat for flying.