Embodied in the Greek version of species was the concept of type, or idea (eidos to the Greeks). Underlying this way of thought is the notion that there is a perfect type that underlies each and every species, much in the same way that geometrical shapes have an ideal. An equilateral triangle is the ideal of all three-sided polygons that we call triangles.
By focusing on type, Greeks, subsequent theologians and pre-Darwinian scholars ignored the interesting differences found among the individuals of a single species.
Variation within a single species is what Darwin realised was important for the processes leading to the origin of species. This was Darwin's seminal contribution to scientific thought.
Prior to Darwin's contribution, the paradigm that people operated under was that species arose by special creation and that species were immutable (unchangeable) after this act of special creation. By asserting that organisms were non-changing, the question of the origin of species or the evolution of species was, de facto, a non-question.
The most famous of these evolutionists was Lamarck. In Lamarck's theory, organisms adapt to their environment by acquiring changes in their lifetime and passing on such changes to their offspring. If such a theory operated in practice, Arnold Schwarzenegger would produce offspring with phenomenal or, at least, above average muscle development, largely because of the characteristics that Arnold acquired during his own youth. This is the theory of evolution by the process of the inheritance of acquired characteristics.
Darwin assumed that organisms naturally vary in almost every attribute that they display. Such variation might lead to differences in survival or reproduction. Because an excess number of progeny are produced in all organisms, there is a competition amongst them to produce successful progeny, or what Darwin called a "struggle for existence."
New species arise from old species by the acquisition of slow changes in traits, behavioural traits included, and such changes are driven by the blind force of natural selection. Mutations arise in a probabilistic fashion. Sometimes the mutations are beneficial, but, more often than not, the mutations are detrimental and such detrimental mutations are weeded out by natural selection.
Natural selection has a tendency to sift out those beneficial mutations that tend to arise only rarely in a population, and to eliminate detrimental mutations.
Darwin formulated his ideas concerning natural selection over the course of many years. The key events in the development of the theory of evolution by the process of natural selection were his voyages on the Beagle. The observations that he made on that voyage generated raw natural history observations on species.
Upon returning to England, Darwin began to develop his ideas on evolution in several "sketch books". From these books he developed the kernel of his theory, the theory of natural selection, by the year 1838. He held on to these ideas for nearly 20 years before publishing them, and he only moved in 1858 because of the fear of being scooped.
Karl Marx used Darwin's theory of the law of development of organic nature for his ideas on the law of development of human history. Marx dedicated a copy of Das Kapital to Darwin. The notion of survival of the fittest and genetics was used to rationalise the eugenics movement, a field focused on the improvement of the human gene pool. The Nazi party in Germany during the 1930s is the most notorious example of the eugenics movement, which resulted in the death of millions of humans. However, the movement was worldwide in scope. Even in the United States, some states had sterilisation laws for "feeble-mindedness" until the late 1950s.
In a reaction to eugenics, Lysenkoism arose to prominence in the Soviet Union. Lysenko was an agricultural adviser of Stalin and had neo-Larmarkian views on the role of environment and species change, and these views dominated Soviet agriculture through the 1950s.
Modern ideas arose concerning the "selfish gene" in human evolution and society. More recently, we have seen the emergence of the discipline of evolutionary psychology, which applies the ideas of behavioural ecology on animals to the human species.
The study of behaviour in all animals, however, can find no room for value judgements regarding a particular pattern of behaviour.