Trees are significantly less of a thing than fish! Fish are almost a phylogenetic class; just remove reptiles, amphibians, birds, mammals, and insects! It's like measure, there are only countably many discontinuities. Trees are more like fins then fish. There are pairs of trees whose last common ancestor wasn't a tree, and visa versa. Many non-trees are just a few genetic tweaks away from being trees, suggesting evolution has kept around a master control switch in case the environment favors trees one day. Trees are...

...what was the original subject? Trees are just so cool