In the material world, there are plants, animals, and humans.
For the plants there is no karma, they are not subjected to the law of action and reaction (karma). There is no greed, ego, envy, hatred etc.. among the plants. There is no ability to question and no God and religion for the plants. The souls in plant bodies are on a fixed transmigration from one plant species to another. Once a soul enters the plant kingdom, it has to go through each of the 2 million plant species (or bodies).
For the animals it’s very similar to the plants, but the animals have much higher consciousness, they can move, and reproduce differently than the plants. Once a soul enters the animal kingdom, it has to go through each of the 6 million animal species (or bodies).
The humans are subjected to the law of karma, for every action taken there will be reaction in proportion to the action. There is greed, ego, envy, hatred etc.. among the humans. There are 400,000 species of human beings and most of them are uncivilized. One can say the souls in plant and animal bodies were once in human bodies, and some souls in human bodies will once again enter the plant and animal kingdoms.
“Those who are envious and mischievous, who are the lowest among men, I perpetually cast into the ocean of material existence, into various demoniac species of life.” (Lord Krishna, Bhagavad-Gita 16.19)
For example if a human being behaves badly in life (maybe one or many lives), then the soul will enter the plant kingdom and go through the 2 million plant species, and then the soul will enter the animal kingdom and go through the 6 million animal species. Then once again the soul will enter the human kingdom by taking birth in a human body. That’s why it is said, a human birth is very rare.
“When one dies in the mode of passion, he takes birth among those engaged in fruitive activities; and when one dies in the mode of ignorance, he takes birth in the animal kingdom.” (Lord Krishna, Bhagavad-Gita 14.15)
The variety of life or species and it’s situations is due to the variety of actions taken and consciousness developed by souls in human bodies.
“The living entity in material nature thus follows the ways of life, enjoying the three modes of nature. This is due to his association with that material nature. Thus he meets with good and evil among various species.” (Lord Krishna, Bhagavad-Gita 13.22)