The walk was continued a couple of hours or more, but no additional prizes were secured. There were not many birds among the trees, and the Doctor said it would be useless to look for deer or similar game on the island, as none were at all likely to be found there. The animals couldn't swim there from the main-land, on account of the crocodiles, and there was no probability that the natives would bring them off in boats, and endeavor to start a deer-park on private account. There were few places where walking was at all easy, owing to the abundance of vines and brushwood. There were occasional patches of grass, but it grew so high that half the time the travellers could not see at all, and therefore hunting was quite out of the question. Soon after their return to camp our friends went on board the steamer for dinner. They came on shore again as soon as the meal was over, and prepared to pass the night on land. The evening brought clouds of mosquitoes that threatened to resent the invasion of the island by devouring the invaders. But the nets had been spread for their reception, and they were disappointed in their hopes, if any had been raised, of tasting American blood. Fred incautiously opened his net to enter it, instead of crawling underneath, according to the approved fashion; the result was that he had music all through the night, which had not been bargained for; but as he was wearied with the walk and excitements of the day he soon fell asleep, and allowed the mosquitoes to sup as they liked. The next morning Fred took a stroll along the shore of the lake in a direction opposite to that he had taken when hunting cranes and king-fishers, and it resulted in an important discovery. He found a field, or badly kept garden, of yams and sweet potatoes. He was not quite certain as to the former, but there was no doubt about the sweet potatoes. He ran back to camp to tell the Doctor what he had found, and the news was immediately sent to the captain of the boat. While they waited for the captain to join them the Doctor explained to his nephew what the yam was. [Illustration: CENTRAL AFRICAN YAM.] "It is," said he, "the popular name for a considerable number of plants of the genus _Dioscorea_, and in the southern part of the United States is applied to light-colored varieties of the sweet potato. The yam is, practically, a tropical plant, though some of its species are found in the middle of the temperate zone. In the latter case the tubers or roots are small and of little value, while the tropical ones often reach a weight of thirty or forty pounds. It contains a large amount of starch, and in its component parts it greatly resembles the potato." Fred asked why it was not possible to have the yam take the place of the potato in the United States, if it grew so large and was such a good article of food.