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active adult amoeboid amphibians anabolic asexual reproduction associated become birds body buds cell-division chromatin ciliated coelenterates colony colour conjugation continuous contrast crustaceans Darwin degenerate dichogamy differentiation division ducts Edited egg-cell eggs embryo emphasised essential evolution extrusion fact factors favour female cells female organs fertilisation fertilised ovum fishes flowers frog functions germ-cells germ-plasma germinal vesicle growth habit heredity hermaphroditism Hertwig higher animals hydroid illustrations important increase individual infusorians insects less male and female male elements mammals maturity medusoid multiplication natural selection naturalists normal nuclear nucleus nutritive observed occur offspring origin ovary ovum parasitic parent parthenogenesis parthenogenetic passive physiological plants polar globules predominance preponderance primitive produce protoplasm Protozoa recognised regard reproductive cells reproductive elements 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 variations vegetative Weismann yolk young
Page 275 - Gavest thou the goodly wings unto the peacocks? Or wings and feathers unto the ostrich? Which leaveth her eggs in the earth, And warmeth them in the dust, And forgetteth that the foot may crush them, Or that the wild beast may break them. She is hardened against her young ones, As though they were not hers...
Page 38 - I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution for the future.
Page 9 - To sum up on the means through which, as far as we can judge, sexual selection has led to the development of secondary sexual characters. It has been shown that the largest number of vigorous offspring will be reared from the pairing of the strongest and best-armed males...