Common fundamental features can be taken into account for classification into certain categories or sub-categories. For the common fundamental feature of all the Arthropods is joined legs for locomotion. Higher animals, like mammals too have joined legs but the difference is the absence/presence of muscles to facilitate articulation. Similarly, all birds have fore-limbs modified to assist in flying. This gives us one clue to categories a particular animal among Avis.
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The animals cannot be classified into different groups,class etc if their common fundamental features are not taken into account .