Fertility has to do partly with the degree of animality in the consciousness. The lower animals breed a lot, whereas the higher an animal rises in the chain, the lesser it breeds. Refinement often comes by drawing/sublimating the energies of procreation into higher intellectual or deeper emotions. That is why the refined races tend to become rarer while the cruder kind multiplies.
However, the so-called extinction serves two purposes. Firstly, it pulls the many upwards towards the vacuum created by the type. Secondly, the refined ones are taken up further towards a higher spiritual consciousness and are seeded everywhere in the course of rebirth to help the general mankind. It is like the pyramid. Finally, in the end, it is the quality that matters rather than the quantity.
Affectionately,
Alok Da
