Life and death, heredity and evolution in unicellular organisms / by H.S. Jennings.

  • Jennings, H. S. (Herbert Spencer), 1868-1947.
Date:
[1920], ©1920
Catalogue details

Licence: Public Domain Mark

Credit: Life and death, heredity and evolution in unicellular organisms / by H.S. Jennings. Source: Wellcome Collection.

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    facts leave readily open the possibility of the building up of a character by minute graded changes. In essentials, therefore, the study of mutations, when carried so far as in Drosophila, is not in disagreement with our observations of gradual variation in the Protozoa nor with the conclusions of palaeontologists as to the gradual de- velopment of the characteristics of organisms in past ages. These conclusions of the palaeonologists are well stated in the recent work of Osborn (1917). He sets forth that in following given stocks from earlier to later ages, characters arise from minutest beginnings and pass by continuous gradations to a highly developed condition. This seems in agreement with the experimental results on both higher and lower organisms, as I have tried to set them forth. The palasontogolical evidence, he holds further, indicates that the hereditary changes as one passes from age to age do not oc- cur in random directions, but follow a definite course, which might seem to have been predetermined in the constitution of the organisms, or otherwise. In the experimental work on the lower organisms little that indicates this has thus far been observed. By selection we can move in more than one direction; though it is also true, of course, that the varia- tions possible are limited by the constitution of the organ- ism. The experimental work has hardly gone far enough to offer important evidence on this problem. There is one other point in the work on higher organisms that we may briefly consider. This is the point made by Bateson (1914) in his Presidential Address before the Brit- ish Association, and further developed in a recent paper by Davenport (1916). It is the paradoxical proposition that since practically all observed variations are cases of loss and disintegration, we are driven to suppose that evolution has occurred by loss and disintegration. Davenport com-