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A great idea is understood in depth when you discover their relationships with previous ideas and similar observations.
There are small, hereditary variations between individuals of a population, but how such a strong differentiation in closely related species can be explained?
An hereditary variation that improves the odds of survival of a species will tend to expand between its population.
“I had been deeply impressed by the manner in which closely allied animals replace one another in proceeding southward over the continent. It was evident that such facts could be explained on the supposition that species gradually become modified; and the subject has haunted me.”
— Charles Darwin
Species evolve and differentiate during successive generations as a result of the fight for survival and the adaptation to the environment through hereditary variations.
“That the increase of population is necessarily limited by the means of subsistence, That population does invariably increase when the means of subsistence increase, and, That the superior power of population is repressed, and the actual population kept equal to the means of subsistence, by misery and vice.”
— Thomas Robert Malthus
“Almost every component in modern man’s belief system is somehow affected by Darwinian principles.”
— Ernst Mayr