The American Naturalist Volume 51

The American Naturalist Volume 51

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 edition. Excerpt: ...separate in the reduction division of maturation, a phenomenon discovered in Drosophila by Bridges (1913) and named by him non-disjunction, may also be the cause of occasional evolutionary changes. The foregoing chromosomal irregularities, which have the appearance of being mechanical rather than chemical phenomena, are not, however, necessarily the instruments with which jiermanent changes of organization are wrought. Probably they are not the usual ones. They have been mentioned first because there is evidence that such changes are occurring now. Minute changes far below the present limits of visibility are as conceivable, and in my opinion quite as probable, as the grosser ones named. In what these changes occur no one knows, for no one knows the nature of the hereditary elements. Suggestions involving enzymes and side chains have been made. These are only conjectures, but they reveal a belief that the phenomena of inheritance are chemical phenomena. If we believe that heredity is dependent upon chemical processes, there seems to me no escape from the assumption that evolution is first of all a chemical change. What the cause of these changes may be is another question; but if changes in considerable fragments of chromosomes, or even in whole chromosomes, can occur as a result of agencies within the organism, as is plainly the case in Drosophila to-day, there is no reason to deny that the invisible modifications of chromosomes, if such occur, are likewise of internal origin. In suggesting possible sources of internal change resulting in evolution I am not blind to the fact that an ultimate explanation of the method of evolution is not thereby offered. The chemical processes which cause these phenomena, while they are distinctly more

Product details

  • Paperback | 234 pages
  • 189 x 246 x 12mm | 426g
  • United States
  • English
  • black & white illustrations
  • 1236863291
  • 9781236863294