What are the motifs in Foundation and Earth by Isaac Asimov?

Diversity is an important idea in this book. Not only are the people and societies on the various planets diverse, but the very core of the reason Trevize is searching for Earth. Trevize has decided that humankind should become a single, superorganism with a single, shared consciousness. Gaia is the beginning of that culture and each individual will become part of the greater whole. There will still be individual people, animals, plants, insects and even inanimate matter, but each will share senses, thoughts and memories with all the others. One of Trevize's biggest concerns is that he's essentially giving up the right for people to be diverse.