Thinking about it... I think Rangers are the generic one's here. Phrasing, to explain, Clerics draw on divine power (generic) whereas Paladins draw on divine power, or on their oaths, but it is fully this fervor and belief that drives paladins (specific)
I think Rangers are the generic ones in this relationship. They draw on natural, primal magic. Maybe from a wide variety of sources and being a general "spirit friend". Meanwhile, Druids go through a specific style of ritual to bind their soul to the forces of the natural world. The specifics of the ritual might change, but all Druids are bound to the power of the planet they are on. This explains why every single druid can turn into animals.
This also sort of makes sense meta-textually. Clerics serve many different gods and have many different powers, but Druids ALL serve "Nature" and mostly have the same powers, their only differences coming from which ASPECT of nature they are particularly embodying. Paladins become more specific because every paladin has a unique drive of morals and goals, seperate from any god or divine power, paladins are in general highly unique individuals. Rangers though are the more generic group, covering a wide range of abilities and motivations.