Darwin hypothesized that favorable traits spread through as species as

It wasn't that the favourable traits were passed down through generations of species, it's that natural selection removed the species who simply couldn't survive. Here's an example: There are two species of moth in a forest, one is brown and the other is black. The trees in the area have trunks that are brown like the moth's wings, so it can camouflage very well compared to the black moths. Eventually, more black moths were being eaten by various amphibians and bugs than brown moths. So, the favourable trait of brown wings continued to be passed on, whereas the black wings went extinct as they were all eaten. Hope that helped :D