Going with what brother Eli Soriano explained, who would see that this particular chapter in the Bible MARK 19-20, does not appear even in the CODEX SINATTICUS which happens to be one of the oldest Bible in the world and precisely it was from the 4th century which implies that it has been written far long from now,
Now comparing MARK 16:15 to the book of MATTHEW 28:19 would see that God was particular about spreading the gospel to Nations and not creatures this time, this would let us know that we can preach the gospel to we humans because it is humans that are made of nations not animals and plants, however the Gospel spreading wouldn't mean one on one preaching or it could also be so however it can be done through media, internet and so on.. Am really glad Brother Eli Soriano as explained this topic in it's simplest form