Perhaps the greatest of Livingstone’s scientific discoveries during this journey was that “of a physical condition of the earth’s surface in elevated tracts of the great continent, unknown before.”  The bogs or earth-sponges, that from his first acquaintance with them gave him so much trouble, and at last proved the occasion of his death, were not only remarkable in themselves, but-interesting as probably explaining the annual inundations of most of the rivers.  Wherever there was a plain sloping toward a narrow opening in hills or higher ground, there were the conditions for an African sponge.  The vegetation falls down and rots, and forms a rich black loam, resting often, two or three feet thick, on a bed of pure river sand.  The early rains turn the vegetation into slush, and fill the, pools.  The later rains, finding the pools already full, run off to the rivers, and form the inundation.  The first rains occur south of the equator when the sun goes vertically over any spot, and the second or greater rains happen in his course north again.  This, certainly, was the case as observed on the Zambesi and Shire, and taking the different times for the sun’s passage north of the equator, it explained the inundations of the Nile.

Such notices show that in his love of nature, and in his careful observation of all her agencies and processes, Livingstone, in his last journeys, was the same as ever.  He looked reverently on all plants and animals, and on the solid earth in all its aspects and forms, as the creatures of that same God whose love in Christ it was his heart’s delight to proclaim.  His whole life, so varied in its outward employments, yet so simple and transparent in its one great object, was ruled by the conviction that the God of nature and the God of revelation were one.  While thoroughly enjoying his work as a naturalist, Professor Owen frankly admits that it was but a secondary object of his life.  “Of his primary work the record is on high, and its imperishable fruits remain on earth.  The seeds of the Word of Life implanted lovingly, with pains and labor, and above-all with faith; the out-door scenes of the simple Sabbath service; the testimony of Him to whom the worship was paid, given in terms of such simplicity as were fitted to the comprehension of the dark-skinned listeners,—­these seeds will not have been scattered by him in vain.  Nor have they been sown in words alone, but in deeds, of which some part of the honor will redound to his successors.  The teaching by forgiveness of injuries,—­by trust, however unworthy the trusted,—­by that confidence which imputed his own noble nature to those whom he would win,—­by the practical enforcement of the fact that a man might promise and perform—­might say the thing he meant,—­of this teaching by good deeds, as well as by the words of truth and love, the successor who treads in the steps of LIVINGSTONE, and accomplishes the discovery he aimed at, and pointed the way to, will assuredly the benefit[73].”