The trigger that tells the hummingbird how to build a nest is some genetically linked mechanism that has been filtered and passed down through the generations. This mechanism (behavior) has likely been perfected throughout these generations so that the hummingbird automatically knows- or senses- when the nest is correctly made. The four things that Darwin noticed are as follows: “every organism produces more seeds or offspring than will actually survive to adulthood”, “there is variation among these seeds or offspring”, “traits are passed down from one generation to the next”, and “in each generation the survivors succeed- that is, they survive- because they possess some advantage over the ones that don’t succeed, and because they survive, they will pass that advantage on to the next generation. Over time, therefore, the incidence of that trait will increase in the population.” While these traits are perfected and fixed, diversity still must be maintained When two unrelated parents combine genes through their gametes, an unlimited number of genetic combinations happens. Through this process, every offspring produced will have a different combination of these genes till the entire community is individually diverse. Fitness for whatever environment or situation and organism is in cannot be obtained if the best (most fit) genes do not get passed on to the next generation.