Episode 21 visuals

A gravid Amazon molly caught near Corpus Christi, TX (© Jon McIntyre)

A female hybrid (left) and a tiger salamander (Ambystoma tigrum) found in Ohio (photo by Katy Greenwald/Ohio Biodiversity Conservation Partnership)

Caricature of Darwin's On the Origin of Species, published in a Punch almanac after the 1881 publication of The Formation of Vegetable Mould through the Action of Worms