James Blish 
Night Shapes [EPUB ebook] 

The continent lay before them, vast areas of it unexplored, its forests, plains, jungle and mountains teeming with forms of life unknown to modern man. Here the witch-doctors reigned supreme, using their inexplicable and remarkable powers on men and beasts. The purpose of the safari was mysterious, its members an oddly assorted group of people unlikely to have any sane objective in common . . .