Question

What evidence does zinn offer to support his assertion that "colonies, it seems, were societies of contending classes"? why is this significant?

Answer 1

In his book, A People's History of the United States, Howard Zinn cites examples from US colonial history of the gap between rich and poor in colonial life.  
A key study cited by Zinn examined tax registers from Boston, showing that the top 1% of the population held 25% of the wealth in 1687, and that by 1770, the top 1% of property owners in Boston owned 44% of the wealth.  The study also noted that the bulk of Boston's population were not property owners.  The percentage of adult males in Boston who owned no property doubled between 1687 and 1770 (from 14% to 29%).
Zinn cited additional items, regarding overcrowding of poorhouses (giving a notable example from New York) and a general increase throughout the colonies of the "wandering poor" who had no real means of support.  He also cited examples of workers' strikes against employers in the colonies because of low wages.