In an article, In a nutshell: Why eastern Jerusalem, and Judea and Samaria are Jewish, Arlene Kushner writes:
Claims by Palestinian Arabs that they were an indigenous people, on the land for many generations, is also a misrepresentation. There is solid documentation for the fact that a substantial part of this group, identified only as part of the Arab nation, migrated into Palestine in the years shortly before the founding of Israel.
One source of documentation is The Late Great State Of Israel: How Enemies Within and Without Threaten the Jewish Nation's Survival , by Aaron Klein.
Below is an excerpt from pages 116-117 in the section A Different Kind Of Refugee. The key point Klein makes is that the unusual definition of refugee applied solely to Palestinian Arabs discards the requirement of "habitual residence" to a mere 2 years. This is in recognition of the large influx of Arabs into then-Palestine because of the improved conditions created by Jewish immigrants.
Klein quotes Mitchell Bard, who writes about the large influx of Arabs into then-Palestine in the years prior to 1948:
The Jewish population increased by 470,000 between World War I and World War II, while the non-Jewish population rose by 588,000.13 In fact, the permanent Arab population increased 120 percent between 1922 and 1947.14
This rapid growth was a result of several factors. One was immigration from neighboring states — constituting 37 percent of the total immigration to pre-state Israel — by Arabs who wanted to take advantage of the higher standard of living the Jews had made possible.15 The Arab population also grew because of the improved living conditions created by the Jews as they drained malarial swamps and brought improved sanitation and health care to the region. Thus, for example, the Muslim infant mortality rate fell from 201 per thousand in 1925 to 94 per thousand in 1945 and life expectancy rose from 37 years in 1926 to 49 in 1943.16
The Arab population increased the most in cities where large Jewish populations had created new economic opportunities. From 1922-1947, the non-Jewish population increased 290 percent in Haifa, 131 percent in Jerusalem and 158 percent in Jaffa. The growth in Arab towns was more modest: 42 percent in Nablus, 78 percent in Jenin and 37 percent in Bethlehem.17
For an example, Elder of Ziyon posts images newspaper articles on the influx of Arabs during the 1930's100,000 illegal Arab immigrants from 1928-1931 and 25,000 from Syria in 1934.
In an article, 1948, Israel, and the Palestinians, Efraim Karsh notes:
the decisive Jewish contribution to Mandate Palestine’s socioeconomic well-being. The British authorities acknowledged as much in a 1937 report by a commission of inquiry headed by Lord Peel:
The general beneficent effect of Jewish immigration on Arab welfare is illustrated by the fact that the increase in the Arab population is most marked in urban areas affected by Jewish development. A comparison of the census returns in 1922 and 1931 shows that, six years ago, the increase percent in Haifa was 86, in Jaffa 62, in Jerusalem 37, while in purely Arab towns such as Nablus and Hebron it was only 7, and at Gaza there was a decrease of 2 percent.
Fred M. Gottheil, in The Smoking Gun: Arab Immigration into Palestine, 1922-1931, goes through the available statistics on Arab immigration.
In Whose Palestine?--appearing in Commentary Magazine in 1986--Erich and Rael Jean Isaac, who critique Joan Peters' From Time Immemorial, also assess the evidence in favor of the case she makes for Arab immigration into then-Palestine:
...there is overwhelming evidence, some of which (for example, in the studies of Fred Gottheil) she uses in her book, of extensive in-migration from the predominantly Arab to the Jewish-settled areas. Scholars, Porath included, do not dispute this (Porath disagrees on the reason for the migration). Such dispute as there is concerns the amount of illicit Arab immigration. The projections do not address this question, but rather confirm the disproportionate growth of areas of Jewish settlement compared with mainly or purely Arab areas within Western Palestine.
Arieh Avneri, in The Claim of Dispossession, published after Miss Peters's book, provides additional data in support of her thesis, with regard both to Arab in-migration and to Arab immigration. (It is noteworthy that Porath, who so vigorously disputes Miss Peters, is one of those thanked by Avneri for "valuable comments" on a manuscript that reaches the same conclusion as hers.) Avneri finds that between 1922 and 1947, in 35 regions of Western Palestine that became Israel, the Arab population increased by 134 percent. By contrast, in 13 regions where there was no Jewish settlement, the Arab population increased by only 98 percent. Avneri points out that even the 98-percent increase is deceptive, for it includes Arab Jerusalem whose population grew over a twenty-five-year period at a rate second only to that of Haifa (150 percent as compared with Haifa's 290 percent). Cities remote from Jewish development grew much more slowly: Nablus, 56 percent; Jenin, 78 percent; Hebron, 64 percent. (Gaza was an exception to these very low rates.)
The rural Arab population also grew in response to Jewish development. The growth was highest in the hinterland of Jaffa, which was the rural area of greatest Jewish concentration, but in the Haifa and Acre district Arab rural population also increased in response to the growing urban demand for vegetables and fruit. In contrast, the rural population in the districts of Jenin, Nablus, Hebron, and Gaza, all remote from Jewish settlements, grew at rates below the national average.
The geographer Avraham Brawer, in his book Eretz Yisrael, published in 1949, compares the population of Western Palestine with that of neighboring countries. He finds that even the purely Arab areas had a population density (96 per square kilometer) equal to that of Lebanon with its more favorable climatic conditions and large, culturally more advanced Christian population component, and double that of the settled areas of Syria and Cyprus, both of which enjoyed better climatic and soil conditions. In the areas of Jewish settlement, the population density was much higher: 136 per square kilometer. Brawer attributes the high population density in all of Western Palestine to the infusion of Jewish capital and the dramatic improvements in public health, which had no equal at the time in any Mediterranean country except France.
The bottom line is: the claim of an established Arab Palestinian land is a myth--the numbers of Arabs in the land fluctuated and rather than "Zionists" being responsible for forcing Palestinian Arabs out of the land, these very same Jews were the reason for many of these Arabs coming into then-Palestine.
UPDATE: Elder of Zion, whose original posts on this topic were very helpful, has another post: More data points on illegal Arab immigration.
I had looked at this in the past, but today I discovered an intriguing new data point, from the Palestine Post, August 19, 1935, quoting the (then Manchester) Guardian of August 10.
The article is a synopsis of the British Treasury report dealing with Palestine. According to the article, Jewish immigration had vastly increased in the early 1930s, but then it adds this:
"The immigration, however, is not restricted to Jews. There has been a steady infiltration into Palestine of Arabs from Syria (the Hauran) and from Trans-Jordan. And it is notable that the illicit immigration of the non-Jews recorded in the report of the Government is more than double that recorded for the Jews."