Why is Zen/Chan thought to be more related to Buddhism than to Taoism?

According to the book [The Tao of Zen](http://en.wikipedia.org/wiki/The_Tao_of_Zen) , Zen/Chan is esentially a form of Taoism. This makes sense to me, as it's hard to understand what is the common ground between the relative simplicity of Zen and the baroque belief systems of the other Mahayana traditions. However, this is not a predominant opinion about the origins of Zen. I understand that it is generally believed that Zen is a branch of Buddhism, with some Taoist influences. What are the arguments that support this opinion? Has anybody provided counter-arguments to the ones presented in *The Tao of Zen*?