The Evolution of Sex

according active already anabolic animals appear asexual associated become beginning birds body called changes characters common comparable conclusion conjugation connection continuous contrast Darwin determination differentiation distinct divides division eggs elements embryo essential explained expression fact favour fertilisation fishes flowers frequently functions further growth hermaphroditism higher illustrations important increase individual influence insects interesting interpreted katabolic known latter less living male and female marked material maturity natural normal noted nucleus nutritive observed occur offspring organs origin ovary ovum parent parthenogenesis pass period physiological plants polar position possible preponderance present primitive probably problem produce protoplasm Protozoa recent referred regard remain reproductive cells result seems seen selection separate sexual sexual reproduction shown similar sometimes species sperm stage structure suggested tend theory tion true union unite usually variations Weismann young