Describe what darwin observed during his voyage on the hms beagle

I think he saw birds of striking different form and structure to those of Europe and went further to establish that it was due to evolution. 

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water