What I'm getting at is taxonomy. A valid taxon has to include all descendents of the crown group. That means that for monkeys to constitute a valid taxon, apes must be included. Same reason why birds are technically dinosaurs.
A monophyletic clade must include all descendants. A taxonomic group itself can hold anything.
Viruses can also integrate DNA into cells and it sticks there forever sometimes, thus bypassing the tree entirely (making it a network, i.e. no longer acyclic thus no longer a tree).
There is a lot of weirdness in the world, stranger than people have dreamed.:-P
Fair enough. I just belong to the people who require a valid taxon to be monophyletic. (Btw., "clade" already implies monophyleti...city? Monophyleticness?)
Also, shut up about viruses, they make a mess of everything and are beautifully chaotic and I hate them and I love them. xD
I am not a biologist, but the way I was taught was that monkeys have tails and apes do not.
As far as the spelling, "monkeys" is correct.
You may be thinking that you want the plural of monkey, but because it ends in y the ending should become -ies. For example: berry -> berries
However, that rule is a little more complicated, and the ending of monkey is -ey. Because there is a vowel before the y the ending you don't have to change the -y to -ie and instead simply add -s
so, frequently people will conflate monkeys and apes, and use the terms interchangably (that's the left end of the graph), people with a bit more knowledge may be aware of the common definition that monkeys have tails while apes do not (that's the middle part), while those with more knowledge of biological taxonomy argue that, since new world monkeys and old world monkeys share a more distant common ancestor than old world monkeys and apes, if we want to define a term 'monkey' that encompasses both new and old world monkeys, it would have to also include all apes (including humans). so, according to the right side of the graph people, chimpanzees are apes are monkeys (though lots of monkeys are not apes, it's a squares and rectangles kinda thing).