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According to Morgan, the heredity of colour-blindness in man is to be explained exactly in the same way as that of white eye in Drosophila. Colour-blind women are rare, but in the few cases known where such women have married normal husbands the defect has appeared only in the sons, as in the second of the diagrams below. Parents Red-eyed male White-eyed female XR XR x XW YW
Moreover it has lately been shown that variations in temperature are one of the chief causes of organic variation. Morgan and Plough, * for example, have discovered that when a certain fly, called the drosophila, is subjected to extremes of heat or cold, the offspring show an unusually strong tendency to differ from the parents.
It is perfectly true, as Morgan says, that new characters which arise as discontinuous variations in other words, those kinds of variation which are called mutations do not add themselves to the line of already existing characters, but 'change the adult characters without as it were passing through and beyond them. The mutations which Morgan describes in his own experiments on Drosophila illustrate this in every case.
He has not only studied the chromosomes in the gametes of this fly, and made Mendelian crosses with it, but has obtained numerous mutations, so that his work is a very important contribution to the mutation doctrine. Drosophila in the hands of Professor Morgan and his students and colleagues has thus become as classical a type as Oenothera in those of the botanical mutationists.
Samuel Lockwood, secretary of the New Jersey State Microscopical Society. His subject was the Wine Fly, Drosophila ampelophila. The paper was a contribution to the life-history of this minute insect. He had given in part three years to its study, beginning in September, 1881, when nothing whatever of its life-history seemed to have been known. In October the flies attacked his Concords.
No mutationist has yet produced by breeding experiments a caterpillar without the three pairs of thoracic legs and yet developing into a moth that had normal three pairs. Morgan, with all his mutations of the adult Drosophila, says nothing of mutants possessing legs.
If the mutation were due to the loss of one factor affecting the eye, the heterozygote carrying the normal factor from the mother only might very well develop a somewhat imperfect eye. Morgan arranges the numerous mutations observed in Drosophila in four groups, corresponding in his opinion to the four pairs of chromosomes occurring in the cells of the insect.
Does this metamorphosis take place in the blind Drosophila of the milk-bottle? The larva of the fly is, I believe, eyeless like the larvae of other Diptera, but Morgan says nothing of the eye being developed in the imago or pupa and then degenerating. There is therefore no relation or connexion between the mutation he describes and the evolution of blindness in cave animals.
Apples that appear perfectly sound when taken from the tree, will sometimes, if kept, be all alive with them in a few weeks." Baron Osten Sacken informs me that it is a Drosophila, "the species of which live in putrescent vegetable matter, especially fruits." An allied fly is the parent of the cheese maggot.
The sex is thus determined by the male gamete, the X chromosome united with that of the female gamete producing female individuals, while the Y united with X produces male individuals. Professor T. H. Morgan has made numerous observations and experiments on a single culture of the fruit-fly, Drosophila ampelophila, bred in bottles in the laboratory for five or six years.
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