The conditions which initiate invasions are legion. The following are some of the more important: (1) changes in forms and routes of transportation;[51] (2) obsolescence resulting from physical deterioration or from changes in use or fashion; (3) the erection of important public or private structures, buildings, bridges, institutions, which have either attractive or repellent significance; (4) the introduction of new types of industry, or even a change in the organization of existing industries; (5) changes in the economic base which make for redistribution of income, thus necessitating change of residence; (6) real estate promotion creating sudden demands for special location sites, etc. Invasions may be classified according to stage of development into (_a_) initial stage, (_b_) secondary or developmental stage, (_c_) climax. The initial stage of an invasion has to do with the point of entry, the resistance or inducement offered the invader by the prior inhabitants of the area, the effect upon land values and rentals. The invasion, of course, may be into an unoccupied territory or into territory with various degrees of occupancy. The resistance to invasion depends upon the type of the invader together with the degree of solidarity of the present occupants. The undesirable invader, whether in population type or in use form, usually makes entry (that is, within an area already completely occupied) at the point of greatest mobility. It is a common observation that foreign races and other undesirable invaders, with few exceptions, take up residence near the business center of the community or at other points of high mobility and low resistance. Once established they gradually push their way out along business or transportation thoroughfares to the periphery of the community. The commencement of an invasion tends to be reflected in changes in land value. If the invasion is one of change in use the value of the land generally advances and the value of the building declines. This condition furnishes the basis for disorganization. The normal improvements and repairs are, as a rule, omitted, and the owner is placed under the economic urge of renting his property to parasitic and transitory services which may be economically strong but socially disreputable and therefore able and obliged to pay higher rentals than the legitimate utilities can afford. It is a well-known fact that the vices under the surveillance of the police usually segregate in such transitional areas.[52]