@gentryman,
it's been a long time since I studied these things, but as I recall buddhism may be adhering to what is called the 'transmigration of souls' and that allows incarnation as an animal.
reincarnation also is central in the hindu religion, but I am pretty sure they believe they will be reincarnated as only a human being in better circumstances if they have been righteous and worse circumstances if they have been sinful. this has kept most of the population at bay for centuries, because they dont believe there is any hope of changing their circumstances in the present lifetime. this, along with the misrepresentation of the theory of caste, has allowed a certain segment of the population to retain power, wealth, political control over the majority. and I dont mean to hurt anyone's feelings here, in six years I have not met anyone who actually takes advantage of this-it is a select group of VIP's.
from what I read in the original vedantic texts and also certain interpretations of them, karma is simply the rule of action having ramifications. and what I can remember from buddhism is that ceasing all action, desire, etc is the only thing that will stop the cycle of rebirth. stopping good deeds as well as bad, good thoughts as well as bad-sounds impossible, doesnt it?
I think most of these things have been mistranslated and corrupted both deliberately and through ignorance and negligence, just as the bible has, probably infinitely moreso, and we are really left to try and make sense out of them in the light of logic as best we can.