as per Darwin 1871 it is decent with modification. With changing ecology an organism adapt itself and its gene frequency changes, leading to evolution in time.
lamarck believe, evolution happens due to wants and needs of an organism. But daawasho natural selection guided evolution. Today synthetic theory explains that evolution happens due to various factors.
Factors of evolution
Ecology. changes in temperature flora fauna. Change organisms size and traits
Biological. changes due to DNA recombination or genetics - foxp2 or notch2nl - that lead to language and brain development in humans.