Khan Academy
Khan Academy: The Chromosomal Basis of Inheritance
Learn about Thomas Hunt Morgan's experiments with the fruit fly (Drosophila melanogaster) as a model system.
Khan Academy
Khan Academy: X Linked Inheritance
Find out about the chromosomal basis of sex determination, X and Y chromosomes, and X-linkage.
Talk Origins Archive
The Origin of Species, Chapter 14
The entire text of Charles Darwin's book, The Origin of Species is available on this site. This link is specifically for the chapter that discusses embryology as evidence for evolution.