How does Hinduism explains reincarnation?
How would it explain the growth in human population? Even if we consider that those humans could have been some other animals in previous life then are we saying that total number of life forms (animals, fishes, insects etc) have remained constant since the inception of earth?
As one of the comments mentioned, reincarnation is not limited to the earth. One can reincarnate as a god, a gandharva, a mane (pitru/forefather), Prajapati or even as Hiranyagarbha (brahmA) himself.
This is from Brihadaranyaka upanishad 4.4.4 with Shankara's commentary -
https://www.wisdomlib.org/hinduism/book/the-brihadaranyaka-upanishad/d/doc122058.html
Verse 4.4.4:
तद्यथा पेशस्कारी पेशसो मात्राम् अपादायान्यन्नवतरं कल्याणतरं रूपं तनुते, एवमेवायमात्मेदं शरीरं निहत्य, अविद्यां गमयित्वा, अन्यन्नवतरं कल्याणतरं रूपं कुरुते—पित्र्यं वा, गान्धर्वं वा दैवं वा, प्राजापत्यं वा, ब्राह्मं वा, अन्येषां वा भूतानाम् ॥ ४ ॥
tadyathā peśaskārī peśaso mātrām apādāyānyannavataraṃ kalyāṇataraṃ rūpaṃ tanute, evamevāyamātmedaṃ śarīraṃ nihatya, avidyāṃ gamayitvā, anyannavataraṃ kalyāṇataraṃ rūpaṃ kurute—pitryaṃ vā, gāndharvaṃ vā daivaṃ vā, prājāpatyaṃ vā, brāhmaṃ vā, anyeṣāṃ vā bhūtānām || 4 ||
- Just as a goldsmith takes apart a little quantity of gold and fashions another—a newer and better—form, so does the self throw this body away, or make it senseless, and make another—a newer and better—form suited to the Manes or the celestial minstrels, or the gods, or Virāj, or Hiraṇyagarbha, or other beings.
Just as a goldsmith takes apart a little quantity of gold and fashions another—a newer and better—form than the previous model, so does the self—these and the preceding words have been explained—again and again crush the five elements beginning with earth and ending with the ether that are always ready at hand, which have been described in the second chapter in the passage, ‘Brahman has but two forms’ (II. iii. 1), and stand for the gold—and make another—a newer and better—form, or body, suited to the Manes, i.e. fit for enjoyments in the world of the Manes, or the celestial minstrels, i.e. fit for their enjoyments, or the gods, or Virāj, or Hiraṇyagarbha, or other beings, according to its past work and knowledge.
Just as humans can become gods and other beings, the reverse is also true. Thus, reincarnation is not incompatible with the growth of human population on earth.
I think you're looking at this all wrong. Obviously my view is monistic, but it's supported by theoretical physics as well as the upanishads. Remember that the reality you think is around you is actually all created inside your mind. There's no such thing as "red". There is some substance that reflects those particular light waves that our visual sense identifies as red. These are the forms existing in a substratum of awareness which our bodies navigate and filter and display as reality to that awareness. There is no specific individual us. There is a gross body, a subtle body, and a causal body all navigating that substratum. Remnants of that causal body suffer rebirth - so we try to get rid of attachments and get those remnants to zero to reach sachcidananda. the self. our ultimate form.
Keep in mind that consciousness is past the 3rd dimension so it is not bound by time. That would mean you're not talking about a linear time progression. So the issue of increased population means squat. We are ALL the same substratum of consciousness. we are just stuck circumnavigating the path of our existing karma until we can zero out and break free.
Life expectancy was longer century back(over 100 years if excluding war deaths) because of cleaner environment and natural foods compared to modern 60-70 years life expectancy and artificial packed foods. Quantity of population is more today but quality is less because of several scientific equipment comforts like vehicles that are cause of polluted air/Prana(life-force).
Is Euthanasia considered morally right?
Today more people are easily handicapped and lethargic because of several equipment comforts and prone to depression, suicide and mental traumas because they fail to use the power of intellect(long term memory) which is also the master of fickle mind, root of mental problems. Even inferior animals dont commit suicide or suffer from depression because of short-term memory, its only with modern humans who are taking pills for depression, turning atheist, committing high suicide rates and wasting precious human birth, because their souls'(of an animal) evolution level is weaker than human body's evolution level as they dont know how to use the intellect, inherited from God in their Man's sheath which Omniscient God itself has used to project this entire creation. Even an animal can be trained to perform certain repetitive actions by its master, so learning to operate devices like phone, computer or vehicle is not real wisdom. Wisdom does not come to a person if he can memorize some complex scientific theory, or get several degrees or operate equipments, its only cramming, real wisdom is when he understands the most the simplest things that is his mind, his purpose of life, humanity and the nature.
So the Jivas go to heaven, and have a very good time, except now and then when the demons give them chase. In our mythology it is said there are demons, who sometimes trouble the gods. In all mythologies, you read how these demons and the gods fought, and the demons sometimes conquered the gods, although many times, it seems, the demons did not do so many wicked things as the gods. In all mythologies, for instance, you find the Devas fond of women. So after their reward is finished, they fall down again, come through the clouds, through the rains, and thus get into some grain or plant and find their way into the human body, when the grain or plant is eaten by men. The father gives them the material out of which to get a fitting body. When the material suits them no longer, they have to manufacture other bodies. Now there are the very wicked fellows, who do, all sorts of diabolical things; they are born again as animals, and if they are very bad, they are born as very low animals, or become plants, or stones.
In the Deva form they make no Karma at all; only man makes Karma. Karma means work which will produce effect. When a man dies and becomes a Deva, he has only a period of pleasure, and during that time makes no fresh Karma; it is simply a reward for his past good Karma. When the good Karma is worked out, then the remaining Karma begins to take effect, and he comes down to earth. He becomes man again, and if he does very good works, and purifies himself, he goes to Brahmaloka and comes back no more.
The animal is a state of sojourn for the Jiva evolving from lower forms. In course of time the animal becomes man. It is a significant fact that as the human population is increasing, the animal population is decreasing. The animal souls are all becoming men. So many species of animals have become men already. Where else have they gone?
