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Updated: June 4, 2025
Natural selection has determined that exogamy produces fitter progeny than endogamy. Cross fertilisation has made stronger individuals and types, and likewise it has maintained them. On the other hand, were family affection stronger than love, there would be much intermarriage of blood relations and a consequent weakening of the breed.
When a gamete with 8 chromosomes unites in fertilisation with a normal gamete with 7 the zygote has 15.
When we examine these somatic differences we find that they can be classified according to their relation to fertilisation and reproduction, including the care or protection of the offspring. The precise classification is of no great importance, but we may distinguish the following kinds to show the chief functions to which the characters or organs are adapted.
Great attention has of late years been paid by naturalists to the wonderful contrivances amongst flowers to secure cross fertilisation; but the structure of many cannot, I believe, be understood, unless we take into consideration not only the beautiful adaptations for securing the services of the proper insect or bird, but also the contrivances for preventing insects that would not be useful, from obtaining access to the nectar.
If eggs are really without structure, the true causes of the formation of birds are the last conditions, whatever they may be, that introduce that phenomenon and determine its character the type of the parents, the act of fertilisation, the temperature, or whatever else observation might find regularly to precede and qualify that new birth in nature.
"The irresistible passion that draws Edward to the sympathetic Otillia, or Paris to Helen, and leaps all bounds of reason and morality, is the same powerful, unconscious, attractive force which impels the living spermatozoon to force an entrance into the ovum in the fertilisation of the egg of the animal or plant the same impetuous movement which unites two atoms of hydrogen to one atom of oxygen for the formation of a molecule of water."
Histology has revealed to us the extremely complicated structure and arrangement of the ganglion-cells. But, for the settlement of this momentous question, the discoveries of the last ten years with regard to the more minute occurrences in the process of fertilisation are of decisive importance.
Yes; but is it not insects that make many of the plants reproductive, and would not whole families of plants die out if their fertilisation was not effected by a class of agents utterly foreign to themselves? No one. The humble bee is a part of the reproductive system of the clover.
The observation of Geoffrey Smith that eggs may occur in the testis of a crab after recovery from the parasite appears of more importance than his peculiar theoretical suggestions, for it tends to show that sex is not always unalterably fixed at fertilisation.
The view above suggested merely attempts to bring our real knowledge of the difference between ovum and sperm into relation with our real knowledge of the sex-chromosomes and their behaviour in reduction and fertilisation. Influence Of Hormones On Development Of Somatic Sex-Characters We have next to consider what are commonly called secondary sexual characters.
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