Hi, LSS. By its very nature, a survey course cannot be thorough. Many interesting topics must be omitted or mentioned only briefly. The Parthian Empire existed for a few hundred years and ruled much of the territory formerly controlled by the Persian and Seleucid empires. It was in regular conflict with the Romans.
I’m not familiar with Collins’s book beyond what’s on its Amazon.com page, but it looks like it attempts to prove an ethnic connection between the Parthians and modern Caucasians. That may be of interest to anthropologists, but it does not necessarily mean that Parthia was important in shaping Western civilization. What’s at stake here?