Your Dugald's father James Drysdale, may be the James Drysdale born 3 Nov 1837 to William Drysdale and Lillias Boyle, in Dumbarton, Dunbartonshire, Scotland. James' next brother was named Islay or Ilay Ferrier Drysdale, born 27 Jul 1840. This Christian name seems significant for your search. Another brother was Romeo Drysdale, born 8 Mar 1831. And yet another was born Dugald McLachlan Drysdale on 2 Sep 1842.