The Origin of Species by Means of Natural Selection: The Descent of Man and Selection in Relation to Sex

adapted admit affinities allied species analogous ancient appear Asa Gray beak become bees believe belonging birds breeds cause cells characters climate closely allied color continent crossed crustaceans degree developed difficulty distinct species divergence doubt effects eggs embryo existing extinct extremely facts favorable fertility fishes flowers formation forms fossil genera genus geological geological periods Glacial period gradations greater number groups of species habits Hence hermaphrodites hybrids important individuals inhabitants inherited insects instance instincts intercrossing intermediate kind lamellæ larvæ less living male mammals manner Marsupials migration Mivart modified descendants natural selection naturalists nearly nest occasionally occur offspring organs parent perfect pigeon pistil pollen present principle probably produced quadrupeds races ranked reciprocal crosses remarked resemblance rudimentary seeds sexual selection slight South America stamens sterility structure struggle successive supposed tend theory tion trees variability variations varieties vary whilst whole widely different wings young