Fletcher Pratt 
The Onslaught from Rigel [EPUB ebook] 

We know that many scientists believe that life may originally have come to earth in the form of spores, from other solar systems and other universes. We therefore might really have had our home dim ages ago, on worlds distantly removed from our earth. The ability to travel the interstellar spaces, however, might also be possessed by other creatures-creatures driven by fear, necessity and by the will to conquer. And if they come, in mighty waves, with scientific powers far beyond us, to dominate the earth, a terrible time will face the puny human race. And in this story they do come, and provoke some of the strangest and most exciting adventures that have yet been recorded…