My own experience was that the nature of the Anichcha, dukka, Anatta related to sense pleasures can be understood without referring to rebirth. In fact I had some sort of exhilaration, like a Eureka moment when I found the real meaning of Thilakkana, at a time “rebirth” was in my ” partially believed, but needs to investigate” list.
There is no problem in understanding the occupants of 31 realms; I believe in opapatika births. Does it mean that the number of beings in the 31 realms is a constant and only the forms will change by transition within the 31 realms? When human and animal populations go higher, will there be a decrease in number of beings in other realms keeping the total number somewhat constant? (I would assume that the number of arahants leaving the cycle be smaller). Within the animal realm itself there will be transition from one form to another, however all animal populations seem to be increasing.
I also understand the gandhabba concept and its relationship with a womb or with an egg, and I do not doubt it. What I am trying to understand is the following.
When I say animals, it includes the whole of animal kingdom. Humans can increase the animal population by farming vigorously and breeding selectively for the purpose thereby changing the nature and the natural instinct of animals. In such a situation, are we to understand it as a Kamma vipaka for those populations of animals? Will it affect the gathi of the animals when living under unfavorable conditions such as in restricted spaces and cages? Then, when, for example animals are slaughtered, often they can see and feel death and naturally it will affect the chutipatisandi moment.
Since humans have the control of certain animal populations can I also assume that there will be countless number of gandhabbas are waiting to get in to the wombs of those animals?
The other question is about lower animals like protozoans; their reproduction is by binary fission.
Then there are animals like Euglena ( also, reproduce by binary fission) that shows characteristics of plants as well, and known as an alga too, can be cultured in a micro-aquarium; there are so many research experiments done on these and other animals and the humans have the control in managing their population. They are also grown commercially for food. What will there rebirth be like?
These questions may sound ridiculous as they may not help the path, but as Buddha said in Kalama sutra, I like to think and clear these for myself as biology is still part of my work. However,I am still at the very beginning of the pure dhamma path, but very much addicted to it. Any help and advice is much appreciated.