What is an evolutionary trend of amphibia?

Ancestor of amphibians were relatives of present day lung-fish/lobefinned fish. The fossil of Ichthyostega is regarded as a missing link between fishes and amphibia.

Ancestors of amphibia developed an air breathing machinary despite having working gills. This helped them to survive periods of drought, hence they received a chance to explore drier habitats.

Two important adaptations of amphibia could be listed as:
1. development of functional pair of lungs,
2. development of four limbs from lobular pectoral and pelvic fins. Limbs may show adaptation to swim in water as well (like webbed feet).

Amphibia's journey on land remains incomplete primarily because of their dependence on water for breeding due to absence of shelled egg. Moreover, their skin remains naked and moist for cutaneous breathing as pulmonary breathing is not enough to supply oxygen to all tissues. Because of these two reasons, they are not able to move away from moist environment.

In fishes, heart receives blood from all organs, sends deoxygenated blood to gills, blood receives oxygen from gills and oxygenated blood is then supplied to body tissues. In amphibia, blood returns to heart from all organs including skin and lungs, hence there is mixing of deoxygenated and oxygenated blood which was not a problem in fishes.

Amphibians still have an aquatic larva in life cycle, which may show neoteny and paedogenesis (larva becomes sexually mature and reproduce). Transitional features of evolution are still present in adaptive features of amphibians for leading a life on land.