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Distinction between the sterility of first crosses and of hybrids Sterility various in degree, not universal, affected by close interbreeding, removed by domestication Laws governing the sterility of hybrids Sterility not a special endowment, but incidental on other differences, not accumulated by natural selection Causes of the sterility of first crosses and of hybrids Parallelism between the effects of changed conditions of life and of crossing Dimorphism and trimorphism Fertility of varieties when crossed and of their mongrel offspring not universal Hybrids and mongrels compared independently of their fertility Summary.
Individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or workers among insects, and in the immature and larval states of many of the lower animals. There are, also, cases of dimorphism and trimorphism, both with animals and plants.
Every pistil, in order to affect fertilization, must receive the pollen from the same length in another flower. Professor Darwin experimented with these flowers and wrote about them to Dr. Gray "I am almost stark, staring mad over Lythrum. If I can prove what I really believe, it is a grand case of trimorphism, with three different pollens and three stigmas.
The deep purple flowers that closely cluster on the long spikes give a rich glow to the lowlands. This flower we found growing in abundance in New Hampshire, New York, New Jersey, and Massachusetts. It is an importation from England. It is remarkable as an example of trimorphism, the two sets of stamens and pistil being of different lengths in the same flower.
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