It would, however, have gratified me exceedingly to have crossed into the Tropic, to have decided my own hypothesis as to the fine country I ventured to predict would be found to exist beyond it. My reasons for supposing which I thought I had explained in my first letter to the Secretary of State, but as it would appear from an observation in Sir John Barrow's memorandum, that I had not done so, I deem it right briefly to record them here. I had observed on my first expedition to the Darling, in 1828, when in about lat. 29 degrees 30 minutes S. that the migration of the different kinds of birds which visit the country east of the Darling during the summer, was invariably to the W. N. W. Cockatoos and parrots that whilst staying in the colony were known to frequent elevated land, and to select the richest and best watered valleys for their temporary location, passed in flights of countless number to the above-mentioned point. I had also observed, during my residence in South Australia, that several of the same kind of birds annually visited it, and that they came directly from the north. I had seen the PSYTACUS NOVAE HOLLANDIAE and the SHELL PARROQUET following the line of the shore of St. Vincent Gulf like flights of starlings in England, and although intervals of more than a quarter of an hour elapsed between the passing of one flight and that of another, they all came from the north and followed in the same direction. Now, although I am quite ready to admit that the casual appearance of a few strange birds should not influence the judgment, yet I think that a reasonable inference may be drawn from the regular and systematie migration of the feathered races. Now, if we were to draw a line from Fort Bourke to the W. N. W., and from Mount Arden to the north, we should find that they would meet a little to the northward of the Tropic, and as I felt assured of two lines of migration thus tending to the same point, there could be little doubt but that the feathered races migrating upon them rested at that point, for a time, so I was led to conclude that the country to which they went would in a great measure resemble that which they had left--that birds which delighted in rich valleys, or kept on lofty hills, surely would not go into deserts and into a flat country; and therefore it was that I was led to hope, that as the fact of large migrations from various parts of the continent to one particular part, seemed to indicate the existence either of deserts or of water to a certain distance, so the point at which migration might be presumed to terminate would be found a richer country than any which intervened. On the late expedition, I accidentally fell into the line of migration to the north-west, and birds that I was aware visited Van Diemen's Land passed us, after watering, to that point of the compass. Cockatoos would frequently perch in our trees at night, and wing their way to the