In a broad sense, human migration is a relatively permanent movement of an individual or a group over a significant distance. Although the significance of a distance is usually measured geographically, it can also be determined by social criteria. For example, a farmer who moves to a city apartment within the same County probably changes his life more ［1］(1. minimally 2. gradually 3. drastically) than does a person who moves from an apartment in New York to one in San Francisco. Taking into account short moves that are socially significant, most analysts agree that a migration must include at least a relatively permanent change of community. Movements that involve only a temporary change of residence are generally considered nonmigratory. These include nomadism, for the nomad has no fixed home, and seasonal movements, ［2］(1. as in the case of 2. unlike 3. aside from) farm workers who follow the growing season. Tourism and commuting also are nonmigratory.
People may move because of dissatisfaction with their community, or because of the attraction of a different community. Examples of the first type of ［3］(1. effect 2. movement 3. incentive) to migrate are the loss of a job, causing the person to consider equivalent employment in another locality, or the exhaustion of natural resources, ［4］(1. including 2. inducing 3. providing) a group to move to a foreign land. These are push factors of migration. On the other hand, a person may choose a new community because its housing or job opportunities are superior, or a relatively large group may suddenly migrate to a distant place when gold is discovered there. These are pull factors. Either kind of motive may operate alone, but these factors generally ［5］(1. interact 2. predominate 3. function) in a migration.
Push factors are more likely to predominate in a less developed country, where families are large and land is scarce. People who can find no means of support in rural areas migrate to the cities in desperate search of work. In developed countries, where opportunities for better housing and employment are more available and where knowledge of other communities is both more abundant and more exact, push and pull factors tend to interact［6］(1. contradictorily 2. more evenly 3.profoundly). Potential migrants weigh the advantages and disadvantages of the region of origin and of the possible regions of destination.
Local migrations are usually motivated by housing and aspects of residential environment such as schools. The chief factor in long-distance migrations is employment, ［7］(1. in that 2. thereby 3. although) differences in housing and climate are the main incentive for elderly people to migrate long distances. The volume of migration tends to be ［8］(1. inversely 2. generally 3. positively) proportional to the distance traveled, because nearby communities are better known. For the same reason the volume of internal migration is greater than that of international migration.
Reciprocal migration takes place when the differences between two regions are evaluated in opposite ways. This ［9］(1. flow 2. difference 3. direction) includes return migration, which often makes statistics on population movements ［10］(1. easier to assess 2. more reliable 3. misleading). For example, up to 40% of the immigrants admitted to the United States between 1890 and 1910 returned home.
Although distance is the chief obstacle to migration, a gold rush, a famine, or religious persecution may produce a sudden large movement to a distant and unfamiliar place. But when the Strong pul or push factor is［11］(1. considered 2. removed 3. reinforced), the volume of subsequent migration in the same direction will depend on interregional evaluations by potential migrants, now better informed. After the California gold rush of 1848-1852and the depletion of the gold, migration to California continued despite the long journey because the state’s basic natural wealth became known and transportation was improved.
The national and linguistic boundaries that pose obstacles to international migration are factors of social distance. Cultural boundaries may inhibit migration even with in a country, as in Belgium across its Flemish-French language frontier. But if two countries have the same language and there is a tradition of migration between them, other social barriers are more easily［12］(1. overcome 2. brought in 3. erected) as in Irish migration to England
Another obstacle to migration is the unwillingness of people to consider moving because of commitments to family, friends, property, and community. ［13］(1. Nonetheless 2. In addition 3. For this reason), the migration rate for older groups is lower than for young adults and their children. Transportation costs are often a great obstacle, and some countries and private enterprises that have［14］(1. restricted 2. promoted 3. rejected) immigration have offered financial incentives to desired new comers.
Current patterns of migration are linked with serious social and economic problems in many countries. In the United States, the movement of millions of people from cities to suburbs and their ［15］(1. increase 2. replacement 3. involvement) in the Cities by people of lower average incomes are causal factors of the modern “urban crisis” The migration from the northern to the “Sunbelt” states has threatened to increase the population of the Southwest beyond the limits of some areas’ water resources. In many third-world countries, rural-urban migration has caused over-urbanization and depopulation of rural areas. The selective migration of highly intelligent, skilled individuals from less to more affluent countries has resulted in the problem of “brain-drain” for ［16］(1. receiving 2. Sending 3. both) countries.
The demographic effects of migration are sometimes difficult to evaluate. The movement of x number of people from Country A to Country B would seem to have the effect of reducing A’s population and increasing B’s by the same amount. But in the long run, this is not necessarily true. Since the migrants typically are young adults, the fertility rate in the sending country may go down, while that in the receiving country may go up. On the other hand, emigration may result in a higher birthrate by ［17］(1. relieving 2. receiving 3. retracting) population pressures that delay marriage and conception. Immigration may lower the birthrate by speeding industrialization and urbanization, thus promoting the upward social mobility of the established population with a consequent ［18］(1. reduction 2. increase 3.balance) in average family size.
When immigrants of diverse cultural backgrounds are assimilated into a new society, they may contribute to the dominant culture. The degree to which assimilation occurs depends on the migrants’ ability and on the receiving society’s willingness to accept them. Assimilation works best where mutual integration is desired, permitting the incoming individuals and groups to remain different［19］(1. such that 2. so that 3. so long as) they do not be come dominant or cause disunity. Where minorities have remained unintegrated or where an attempt has been made to achieve ［20］(1. social welfare 2. mutual independence 3. complete uniformity), cultural collisions have occurred. In the United States the concept of the population as a “melting pot”or uniform blend of human ingredients, has changed since the mid-20th Century. It is now generally recognized that immigration produced something more like a mosaic, in which each element has retained its own characteristics while contributing to a commonality.
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