As I unpacked my possessions from cardboard boxes this week and surveyed the tight-packed gable roofs and garage sheds of my new neighbourhood, I was struck by my own role in ending Canada's postwar immigration miracle.

My wife, my children and I moved this week from the house we'd bought in the 1990s in what was then a poor immigrant district to a slightly larger house we'd purchased, a few hundred metres away, in a Toronto neighbourhood, Christie Pits, that has been the landing pad for generations of impoverished Eastern European Jews, then Portuguese peasants, then East Asian villagers.

Our old house had previously contained three kitchens. Our new house came with three, plus the plumbing, in the attic, from a fourth. We'd purchased it from a Portuguese widow and her daughters for 35 times what she and her late husband, a tradesman, had paid in the 1970s; they had been able to become homeowners by renting most of the house. She and her children had used the proceeds from our purchase to buy houses in the suburbs.

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What had we done? On one hand, we'd propelled several immigrant families into the middle class by allowing them to cash in on their property-value gains. On the other, we'd turned as many as seven units of housing accessible to poor immigrants into two units of housing affordable only to well-employed middle-class people.

For more than a century, dense-packed neighbourhoods like these near the centres of Canadian cities received poor, illiterate but very ambitious immigrants, who took advantage of the small-business opportunities, dense networks of mutual assistance and easy credit based on rising property values and used those factors to improve their lot in life.

I gave this sort of bottom-rung district a name – the "arrival city" – in my book by that title, because they are really cities within cities created by clusters of people from the same regions. But even as I was coining that phrase, I was helping to kill the great Canadian arrival city.

I'm a serial gentrifier: My embrace of the reciprocal saw and the nail gun has made me an agent of social change. Which means two things.

Up to now, gentrification has been a force of good: Because six out of seven immigrant families purchase houses, and studies show that even poor immigrants have usually become home owners, the influx of artisanal coffee boutiques has been a huge gift to those Canadians who were born on mud floors in another part of the world. It's a big part of the reason why the rags-to-riches story is more of a reality in Canada than in other countries.

But gentrification also changes that story. The next generation of immigrants won't be able to follow the Jews, the Portuguese and the Chinese through my neighbourhood's back alleys and into the middle class. No casually employed person, immigrant or otherwise, can afford a house downtown in a big Canadian city any more, even if they put four kitchens in it.

Because of people like me, immigration and poverty are both now almost entirely suburban phenomena. And the suburbs aren't as well suited to be the bottom rung on the ladder: They lack the population density, access to consumers, rapid transportation and small-business opportunities of the old arrival cities.

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As a result, our cities are becoming polarized. Research by University of Toronto urbanist David Hulchanski shows that in downtown Toronto today, six of 10 people are university educated, almost 40 per cent of families earn more than $100,000, 28 per cent are foreign-born and 82 per cent are white. In the inner suburbs, three of 10 are university educated, one in 10 have six-figure family incomes, 60 per cent are foreign-born and 34 per cent are white. Before, the newcomers were in the centre. Today, people like me are there, and the non-white, non-wealthy and foreign-born are on the edges, without much chance of grabbing the bottom rung of the housing ladder. (This is the division that led to the 2010 election of Rob Ford in Toronto: His backers, the non-white, non-wealthy, non-homeowning suburbanites, expressed their frustration with their ballots.)

If we don't want the Canadian dream to be ruined by people like me, we'll need to turn those inner-suburban districts into the sort of arrival cities we used to see downtown. We should take lessons from Surrey, B.C., which has allowed affordable basement and backyard "infill" apartments to flourish, or Toronto's "tower renewal" zoning, which allows dense, high-intensity, multiuse districts to flourish between suburban apartment blocks.

For the past 60 years of immigration, Canada got lucky. In the coming decades, we'll have to get skilled.