For a refresher on the difference between primary sources and secondary sources, click here.
The following may refer you to good primary sources on your topic.
They can also help you identify relevant historical figures authors, or keywords for searching library databases.
For example, in Tim Lewens' book Darwin (shown below), he discusses Darwin's remarks on a "moral sense". In this context, Tim Lewens' book is a secondary source, and his reference (Darwin's Descent of Man) is the primary source. If you wanted to analyze for yourself Darwin's thoughts on morality, you would know now to check out Lewens' references, find this particular edition of Descent of Man in the library catalogue and read around page 120.
Remember when you're reading secondary sources to look for relevant keywords and thinkers. If I was interested in Darwin and morality, I would make sure to jot down ethics, moral sense, biologicized as important keywords to search and EO Wilson as an important person to read up on.
Combine keywords for different kinds of primary sources with keywords for your topic to find them in the library catalogue.
Primary source keywords
evolution early works
Eugenics personal narratives