Sometimes I think humans overanalyze things abit. Fun, isn't it! :-D
In the Garden of Eden, there were basically two things that God gave us to occupy our time: sex and eating. God told Adam and Eve, "Be fruitful and multiply, and since you'll work up an appetite doing that, I've given you all these wonderful plants and trees to eat of." (paraphrased version of Genesis 1:28-29) It can certainly be assumed that the animals were in on this rather cushy assignment as well. For we see the repititon of 'after its kind' (killer phrase for evolution) in the creation story, and in Genesis 1:30 the food is for the animals as well.
Food and sex are necessary to all with nephesh in order to fulfill our prime directive "Be fruitful and multiply." Because these are such basic needs for all living creatures that have nephesh, God, in His goodness, made both of these things pleasurable. And, because both of these things are pleasurable, after the Fall, they are both regulated. There are strict commandments regarding both our sexuality and our eating habits.
These commandments are directed towards people, but animals exhibit behaviors which violate God's commandments. As we see, there are gay dolphins and lesbian Japanese macaques, and the bunnies are just out of control. The animals also eat each other, a direct consequence of the Fall, as all with nephesh were created to be vegetarian. And, as I sit here with my breakfast, after feeding the herd, I can personally attest to the gluttony of even herbivores. If I don't string the hay out far enough, some of the younger ones are in real danger of being gored if they don't back up and let the older ones eat until they are finished. (And, it's not because they are starving, we have the fattest cattle in our valley.)
Genesis 6:11-12 seems to be a key verse: "Now the earth was corrupt in the sight of God, and the earth was filled with violence. And God looked on the earth, and behold, it was corrupt; for all flesh had corrupted their way upon the earth."
I don't think we can assume that all flesh means just all men, because God both destroyed the animals, and yet saved a remnant of the animals. And the first covenant He made after the flood was also with the animals as well as with the humans.
Now, as to Mark 16:15. We need to keep in mind that the context of the passage is the Great Commission, which is to both preach the gospel all nations and baptize all believers. Drew, I know you are sincerely struggling with what this passage means, but I doubt you would suggest that we baptize the cat. (I wouldn't recommend it! :wink: )
I think we can discover the true meaning of this passage by studying the other text regarding the Great Commission. Matthew 28:19 seems to make it clear that Mark 16:15 is simply saying to go into all creation, in other words, throughout the world, making disciples and baptizing them.
One thing to consider is that many of the earliest manuscripts end Marks Gospel at verse 8. Verses 9-14 are only on the later manuscripts. However, it's clear from examining ancient documents that the message in verses 9-14 must have been part of Mark's original Gospel. I think a logical explanation is that somehow or another, the end of the Gospel was lost, and later copyists paraphrased what was known to be included. This could explain why the wording of the Great Commission seems a bit strange to us in Mark's Gospel, but perfectly clear in Matthew's. If you do a study on the language of Mark 16:15, you can see that the verse can just as easily be translated, "preach the gospel to all creation" which can easily mean to "all the earth." Which is exactly how my Bible translates this verse. The KJV is a great translation, but sometimes we can get better clarity with other translations as well.