CAN WE SEE EVOLUTION OCCURRING?
as well as from both of the parents.[1] These differences show themselves in the characteristics of the developed individuals, in thousands of diverse ways, some very marked, some extremely inconspicuous. It becomes therefore extremely difficult to distinguish differences produced in this way—by recombinations of genes in biparental reproduction—from differences that are steps in evolution. In most higher organisms this is indeed at the present time impossible.
Yet in certain higher organisms these difficulties have been overcome. By study, continued for years, the hereditary constitutions of the parents are thoroughly learned, so that the results of their combination are known. In such organisms the hereditary constitution does change at times, irrespective of the recombinations due to the union of two parents. The changes are infrequent. Yet in such an animal as the fruit-fly (Drosophila), studied by Morgan and his disciples, so great has been the number of individuals and of generations minutely studied that literally hundreds of different alterations in hereditary characters have been observed. Drosophila has given rise to hundreds of new stocks, which differ permanently from the original one. Some of the changes are strongly marked, as when red-eyed animals suddenly produce a white-eyed race, or when long-winged creatures suddenly produce a race that is permanently without wings. These very marked changes were naturally the first ones observed, so that for a long time it was believed that all evolutionary changes were large leaps, saltations. But since acquaintance with the animals has become more minute it has been discovered that extremely slight, almost imperceptible, changes in hereditary characters are much more
- ↑ For details as to this process of recombination of genes in reproduction from two parents, see any modern text-book of genetics; e.g., T. H. Morgan’s The Physical Basis of Heredity.
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