Essays upon heredity and kindred biological problems / by August Weismann ; authorized translation edited by Edward B. Poulton, Selmar Schönland and Arthur E. Shipley.

  • August Weismann
Date:
1889
Catalogue details

Licence: Public Domain Mark

Credit: Essays upon heredity and kindred biological problems / by August Weismann ; authorized translation edited by Edward B. Poulton, Selmar Schönland and Arthur E. Shipley. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    caterpillar during the course of its growth determines whether the lighter or darker colour shall be developed. Here therefore we have a case exactly parallel to that of the Thuja-shooi in which the palisade or spongy parenchyma is developed according to the position in which the shoot is fixed. As fer as it is possible in the present condition of our knowledge to offer any opinion upon the origin of sex in bisexual animals, it may be suggested that this problem is also capable of an essentially similar solution. Each germ-cell may possess the possibility of developing in either of two directions, the one I'esulting in a male individual, and the other resulting in a female, while the decision as to which of the two possible alternatives is actually taken may rest with the external conditions. We must, however, include among the externa] circumstances everything which is not germ-plasm. More- over, this explanation is by no means certain, and I only mention it as an instance which, if we assume it to be correct, further illus- trates my views upon the phenomena presented by the TJiuja-shooi. The two other facts brought forward by Detmer as proofs of the transforming power of external influences can be explained in precisely the same manner. These instances are—the fact that Ti'opaeolum when grown in moist air produces leaves with anatomical characters different from those produced when the plant is grown in dry air; and the differences in the structure of the leaves of many plants, according as they have been grown in the sun or shade respectively. Such differences do not by any means afford proof of the direct production of structural changes by means of external influences. How would such an explanation be con- sistent with the fact that the leaves are, in all these cases, changed in a highly purposeful manner ? Or is it assumed that these organs were so constituted from the beginning, that they are compelled to respond to external conditions by the production of useful 1, li, and 1888, p. xxviii), the cocoons of the same species being of a creamy white colour when spun upon white paper. Conversely, the light reflected from the same surfaces serves as the stimulus for ^vith]lold^7}g pigment in the cases alluded to by Dr. Weismann (larvae of B. Crataegata, &c.), in all of which the organism only remains in contact with the leaves while they are green, viz. at a time when the dark colour would be dis- advantageous. Hence precisely opposite effects are produced by the operation of the same force; the nature of the effect which actually follows in any case being solely determined by the advantage afforded to the organism.—E. B. P.]