I recommend „Capital in the 21st Century“ by Thomas Pikkety, an economist who has deeply studied wealth and income equality using historical data spanning three centuries. This book has all the evidence you need.

https://www.amazon.com/-/de/dp/0674979850

Spoiler alert: income from wealth is on its way of becoming close to being as concentrated as it was in the 19th century (especially in the US), and the share of income from work in total national income is decreasing almost anywhere. So yes, increasingly you can only accrue significant wealth by already having significant wealth.

> Citation needed.

Here you go: "The fading American dream: Trends in absolute income mobility since 1940", Raj Chetty et al. https://science.sciencemag.org/content/356/6336/398