It should be borne in mind, moreover, that the area of cultivated land in Hawaii is very small, the amount reclaimable still smaller, while the needs of a growing population must be met. This, of course, means intensive cultivation and a high average rate of wealth production per acre. In the ten-year period from 1900 to 1910, the population increased 24.6 per cent and the area of tillable land 3.6 per cent. The census reports also show that Hawaii is already cultivating its land far more intensively than the mainland states; for example, it supports twenty-two times as many persons per acre of improved arable land as the agricultural state of North Dakota. Clearly, the problem in Hawaii is peculiarly difficult. It is true, also, that practically all tropical industries demand a plentiful supply of cheap labour. Labour in Hawaii is neither cheap nor plentiful. In this respect, the Islands are at a disadvantage compared with nearly all tropical countries, but much money has been spent on the industries, and the results are certainly encouraging. How to secure cheap labour has always been a serious question for the planters. The Bureau of Immigration was established in 1876. When the Reciprocity Treaty with the United States was signed, several thousand Portuguese were sent for by the government and the planters, and many of them have remained in the country and become good citizens. About the year 1888, however, it was decided that the Chinese and the Japanese should be encouraged to come, because the cost of transportation for them was so much less. For some years the larger part of the labourers were of these two nationalities. The Japanese are still far in excess of all others, numbering over 93,000. After annexation, when the Congress of the United States prohibited immigration by the yellow races, Hawaii was obliged to seek a supply from other sources. Filipinos, of whom there are only 8,000, are next in number to the Japanese; Portuguese, Chinese, Spaniards, and Porto Ricans stand next. After the expenses of the voyage were paid, the labourers did not always keep their agreement to work, so contract labour was introduced. Although some objections have been made to the contract system in Hawaii, it must have proved fairly satisfactory to both parties, for in those days a large number of labourers would sign a second contract on the same terms, showing at least that they were well treated and paid according to agreement. In some cases, Chinese and Japanese labourers remained in the Islands after their contract expired, and settled there permanently. Many of the Chinese became merchants. The Portuguese went into fruit raising, and the Japanese kept mostly to the coffee plantations. In those days, the Japanese had labour unions, and they were sometimes troublesome.