The dissimilar but homologous forelimbs in Fig. 22.15 come from an ancestral vetebrate/mammalian forelimb, but have faced different selection pressures for dexterity (human), running (cat), swimming (whale), and flying (bat). The similar but not related gliding animals in Fig. 22.18 have developed analogous structures due to similar selection pressures favoring gliding via convergent evolution.
Work Step by Step
Review the material on homology and analogy. The limbs in Fig. 22.15 have common origins as you can see from the altered but shared bones, color-coded for clarity. The two gliding organisms belong to different groups of mammals with many non-gliders between them, evolutionarily, so they must have convergent structures for gliding.