The one question, I have always had about reincarnation is how do you know you have enough Karma stored up to upgrade to the next form of existence? Who says one life form is a step forward and another is step back. It's the same problem with evolution. A lot animals who are considered "beneath" us have abilities that we don't have. If evolution is all about passing the best traits so that the next level life form has the best chance for survival then why can't we see in the dark as well as cats? Why can't we fly? Don't get me wrong I thank God for my oppoesable thumbs, but if evolution was true would not the highest evolved creatures contain the best traits of all the other animals in the descent tree? I know some would aregue that we are not directly descended from cats or birds. Fine. But how come all the other primates have greater physical strength and durability then we do? Would not the ultimate primate have the strength and staminia of a mountain gorrilla but have human "intellect"? No such being exists on earth. And I see no way such a one could evolve out of us.
Also, concerning reincarnation, if it were true, then the sum number of living things on the earth would never change. I mean if souls are only switching bodies then it would be a closed system. Why then would there be more humans living on earth simultaneously than at any previous time? By extension, I doubt that that the same cannot be said about other species of life, even if you allow for souls to be in any living thing even plants. Therefore reincarnation cannot stand.