Charles Darwin 
The Different Forms of Flowers on Plants of the Same Species 

In The Descent of Man Darwin addresses many of the issues raised by his notorious Origin of Species: finding in the traits and instincts of animals the origins of the mental abilities of humans, of language, of our social structures and our moral capacities, he attempts to show that there is no clear dividing line between animals and humans. Most importantly, he accounts for what Victorians called the 'races’ of mankind by means of what he calls sexual selection. This book presents a full explanation of Darwin’s ideas about sexual selection, including his belief that many important characteristics of human beings and animals have emerged in response to competition for mates.