Is there any possible theoretical explanation for the fact that a Pokemon’s evolutionary line can’t exceed three stages? (Apart from Mega Evolution, which is temporary.) I love your blog, thanks for writing!

Hmm.  My suspicion is that more than three stages would not be advantageous from a natural selection perspective.  Unevolved Pokémon are, in general, vastly more common than evolved ones (even ones like Butterfree that evolve very quickly), which suggests that the majority of them don’t ever make it to their final forms in the wild.  To pull some numbers out of the air, maybe only 5-10% of all Bulbasaur ever become Ivysaur, and only 5-10% of those ever make it all the way to Venusaur – between 0.25% and 1% of the total population.  In order to evolve a hypothetical fourth evolution (…damnit, I hate it when I have to talk about Darwinian evolution and Pokémon evolution in the same sentence), there would need to be a significant selective advantage conferred on (for example) Bulbasaur who possessed the genes for that fourth stage versus Bulbasaur who didn’t – but if any individual Bulbasaur with the appropriate mutations has less than a 0.1% chance of ever using them anyway, then for most of them it won’t make any difference at all.  Essentially, the sheer odds against a wild Pokémon ever reaching its fourth stage make it pointless even to have a fourth one.