Evolution and genetics / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1925
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Licence: In copyright

Credit: Evolution and genetics / by Thomas Hunt Morgan. Source: Wellcome Collection.

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    descriptive embryology. Today it is taught in many textbooks of biology. TIaeekel called this interpre¬ tation the Biogenetic Law. The parallel between tbe historical development of antlers of deer (Jig. 8) and their postnatal development from year to year (Jig. 9) is most striking. Historically we may sup¬ pose that the development was due to the appear- Fig. 8.—Fossil deer antlers. The first two to the left are from the mkl-eosine; the third is from the upper miocene; the fourth is from the pliocene, as is also the fifth figure. The figure to the right is from the “forest-bed of Norfolk.” (After Romanes.) ance of hereditary variations in the germ material. At the present time, the stages in their development are closely correlated with age, size, and especially with the yearly increase in the internal secretion of the testes; for, after castration the antlers no longer develop. In living deer all the hereditary factors that appeared in the past are present at each stage, but the extent to which the antlers develop depends on the physiological conditions mentioned above. In other words, while there is a close relation in both cases
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