The Evolution of Sex

active alternation amoeboid anabolic asexual reproduction associated become birds body buds cell-division chromatin ciliated colony conjugation continuity contrast crustaceans Darwin degenerate differentiation dimorphism division ducts egg-cell eggs embryo emphasised essential evolution expression extrusion fact factors favour fertile fertilisation fishes flowering plants flowers frog functions genesis germ germ-cells germ-plasm growth hermaphroditism higher animals hydroid illustrations important increase individual infusorians insects larvæ less male and female male elements mammals mates maturity Maupas medusoid multicellular multiplication natural selection naturalists normal nuclear nucleus nutritive observed occur offspring origin ovary ovulation ovum parasitic parent partheno parthenogenesis passive physiological plants polar bodies polar globules predominance preponderance primitive produce protoplasm Protozoa recognised regard relatively repro reproductive cells reproductive organs result rotifers secondary sexual characters sex-cells sexual reproduction sexual selection species sperm spermatozoa sponges stage structure theory threadworm tion union unisexual usually variable variations vegetative Weismann worms yolk young