(4) the urbanization bring enormous social demand of industrial cluster.

Market demand is the fundamental cause of industrial cluster. The process of urbanization, is also a city constantly emerging and development process. In this process, the urbanization can produce huge social demand to the national economy, for example, urbanization needs and provide a large number of surplus agricultural products, agricultural industry provide basic construction and funds and provide the perfect service, services, and so on. So the city becomes enterprise provides a favorable area of social production, what is more important for the enterprise put forward the great social demand, prompting companies all for production, capital goods for urbanization. The result is: on the one hand, for the urban construction, urban development provides ample economic basis, like infrastructure, public facilities, such as money, on the other hand, gives country factors of production transferred to city huge economic gravity and opportunities. Due to the existence of the external economy, scale economy, scope economy, enterprise in order to obtain the economic benefits will be in a certain range of the cluster, enterprise cluster provides the personnel, the production elements of the agglomeration gravity and bearing capacity, the enterprise and the population agglomeration and reinforce the role of the external economy, scope economy, scale economy, so as to attract more enterprises, population, production factors, such as moving from rural to urban, and finally form the industry cluster, and the deepening of industrial cluster.

(5) a city into a industrial cluster constant elements and industry transfer.

Urbanization, once you begin, the economic interests of the urban economy and rural economy will continuously expand, this kind of comparative advantage

is derived from the city's external economy, scale economy, scope economy, this kind of comparative advantage will continue to draw the rural population, capital and other factors of production into urban economy to contribute to the development of urban economy, and further intensify the gap between the urban and rural economy, so as to attract more factors of production transferred to the city. At the same time, in order to share the external economy, scope economy, enterprise clusters tend to guide the agglomeration of production factors. Urbanization is a dynamic process, this process will continue to expand the comparative advantage of urban economy and rural economy. So, urbanization can bring industrial cluster of factors of production constant inflows, this is a direct result of the factors of production who pay. Only when the urban economy and rural economy of comparative advantage, factors of production are no longer to cities, then the development of the economy to maintain a balance process, it is both a sign of complete urbanization, is also a mark of success of industrialization. The development of industrial agglomeration, urbanization is the essence of urbanization is mainly the development of industrial clusters. Industrial agglomeration must attach great importance to the city brought about by the huge external economic benefits, make full use of function of urban agglomeration and the economic effect of urbanization, to get the effect of get twice the result with half the effort. Only, on the other hand, industrial concentration should be brought into the overall framework of urban development, on the space around the city or urban area,can more effectively promote the development of urban infrastructure construction and social undertakings, near field to promote more way to accelerate the process of urban development and urbanization.

一、外部性产生了产业集聚的主要向心力

早在马歇尔时代就已经对产业集群进行了研究。马歇尔解释了基于外部经济的企业在同一区位集中的现象。他发现了外部经济与产业集群的密切关系,认为产业集群是外部性导致的。马歇尔关于产业集群理论起源的外部性理论,成为产业集群形成机理分析的经典,被称为“马歇尔外部性”。自从马歇尔(1890)提出产业集群,并指出著名的三原因(劳动市场共享、专业化投入品和技术服务、知识外溢)后,产业集群及其原因、影响就一直成为经济学家研究的重点。不同学派从不同假设条件出发,通过不同的研究方法,得出了不少有意义的研究结果。各学派取得的一致意见是:外部性是产业集聚的源动力。面我们主要从货币外部性和技术外部性两个方面来论述产业集群的外部性。源[自-优尔*`论/文'网·www.youerw.com/