Description and physiological botany / [J.S. Henslow].

  • John Stevens Henslow
Date:
1839
Catalogue details

Licence: Public Domain Mark

Credit: Description and physiological botany / [J.S. Henslow]. Source: Wellcome Collection.

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    species, may be fairly stated as purely hypothetical. Linnseus supposed that only a few species or distinct I typical forms were originally created, and that a mul- 1 titude of others had since been derived from them by » repeated intermixture and crossings. He supposed the species of very different genera might be capable of in- termixing and producing new species, and even new genera. These speculations are wholly unsupported by facts or experiments. De Candolle also supposes a de- finite number of species or typical forms to have been originally created, but he does not imagine any de- cidedly new form or type to have ever originated from them. He considers that certain hybrids can repro- duce their kind, but that in such cases there exists a constant tendency in the offspring to return again into one or other of the original types from which they sprang. Thus we should never have any strictly new type intro- duced, or any form which differed very materially from what was already in existence, but only a multitude of minute shades of difference, in varieties which were all intermediate between the original species. In this way he proposes to account for the endless varieties of some of our long cultivated fruits, as apples, pears. See. The subject is one of great difficulty, and it will require many accurate and careful experiments to be made, before we can expect to ascertain the laws by which the limitation of species and the production of hybrids are regulated. We are quite certain that many forms, con- sidered characteristic of particular species, have con- tinued unaltered in their minutest particulars for the last 3000 years at least. This is proved by a careful examination of the fragments of numerous plants found in the catacombs of Egypt. An analogous fact is still more strikingly established in the animal kingdom, and for a much longer period; since the forms of certain existing species of shells have been found in those ter- tiary deposits of which the geologist can say no more than that they are comparatively recent in the liistory u
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