The Origin of Species by Means of Natural Selection: Or, The Preservation of Favoured Races in the Struggle for Life

accumulated adapted affinity allied species animals aphides appear areas become believe birds breeds Cambrian characters CHARLES DARWIN climate closely allied common continent crossed crustaceans cuckoo Darwin degree descended developed difficulty distinct species disuse domestic effects eggs embryo embryology eminently liable existing extermination extinct extremely fact favourable females fertilised fertility flower formations fossil genera genus geological geological period giraffe gradations greater number groups habits Hence hybrids hybrids produced important increase individuals inhabitants inheritance insects instance instincts intercrossing intermediate intervals islands larvae less living males mammals manner Marsupials migration Mivart modification natural selection naturalists nearly nest neuters occasional offspring organisation Origin of Species parent perfect pigeon plants pollen probably produced progenitor pupae quadrupeds rank reciprocal crosses reproductive systems resemblance rock-pigeon rudimentary organs seeds sexual selection slaves slight stage sterility structure struggle swimbladder tend tendency theory of natural tion variability variations varieties vary widely young