Define “Darwinism”. I’m familiar with the theory of evolution. “Darwinism” seems a slippery term that defies formal definition, other than being a general perjorative.
http://plato.stanford.edu/entries/darwinism/
http://www.merriam-webster.com/dictionary/darwinism
http://www.britannica.com/EBchecked/topic/151986/Darwinism
But like every historical entity, theories undergo change through time. Indeed a scientific theory might undergo such significant changes that the only point of continuing to name it after its source is to identify its lineage and ancestry. This is decidedly not the case with Darwinism.
From this description, the term "Darwinism" seems to have been arbitrarily and unnecessarily assigned, since the basic theory admittedly has not undergone any significant revision or divergence to make it necessary to designate or group the theories by lineage.
As a philosophical designation, it appears to be indistinguishable from philosophical naturalism, which again seems unnecessary, and potentially confusing and ambiguous.