Darwinian evolution is slow and gradual, step by step. Such an evolution can explain micro-evolution but not macro-evolution. For example, how did the eye evolve? The idea behind Darwinism is that organisms adapt, and that nature selects only those genetic changes which are the mutations that serve a good purpose for adaptation. So taken this way, the eye cannot develop gradually because one-thousandth or one-millionth of an eye would be of no value for survival. So generally this question rules out Darwinism as an adequate theory for macro-evolution.