Volume 1
A system of medicine by eminent authorities in Great Britain, the United States and the Continent / edited by William Osler, assisted by Thomas McCrae.
- Date:
- 1907-10
Licence: In copyright
Credit: A system of medicine by eminent authorities in Great Britain, the United States and the Continent / edited by William Osler, assisted by Thomas McCrae. Source: Wellcome Collection.
Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.
57/986 (page 33)
![Once again to take a botanical example: in a bed of single tulips a rare flower-head may show only five petals; another may show seven. The liliacea;, to which the tulips belong, are throughout characteristically six partite, and if one of these variations is reversionary to an ancestral con- dition, the other cannot be. From the fact that in neither case is the general charaeter of the tulip greatly altered, the probability is that neither condition represents a reversion, and that both are shorts. The four-leaved shamrock is of the same order. We owe to the great Belgian botanist de Vries the fullest study and recognition of this production of ‘mutations.Cultivating the plant Oenothera Lamarckiana over a long series of years—in fact since 1886—he observed the appearance from time to time of individuals which dehnitely varied from the parents, and, what is more, when fertile, were true to seed. Thus in 1895, to quote an instance, there appeared the relatively huge Oenothera gigas, and that not by gradual variation but by a sudden jump. Subjected to self-ferti- lization this single plant afforded seeds giving origin to plants (several hundreds) of the gigas type. At a bound therefore a new species was seen to develop. As de Vries points out, we have here an example of discontinuous and not of gradual evolution, and he holds that in all cases evolution must be of this discontinuous type. In this as supporting a physico-chemical theory of inheritance we cannot but agree with him. To quote Jacques Loeb,^ “If the (determinants) are comparable to a series of compounds, e. g., of alcohols, there is no more a transition possible between two species separated by a difference in only one determinant than there is a transition possible between two neighbour- ing alcohols of the same series.” We meet with similar conditions in animals. Thus it is known that the celebrated royal strain of cream- white horses at Hanover originated by careful inbreeding from a single sport—a white horse which appeared fortuitously in a breed that had previously shown no tendency to throw white horses. Certain breeds of merino sheep are also traceable back to a single sport possessing silky hair in place of the ordinary wool; and so on. In man we occasionally encounter the same, though it has to be admitted that well-authenticated family histories are rare, and we cannot always assure ourselves that a given sport has appeared in a given family for the first time; i. e., that it is not an inheritance. The most frequent examjdes are what Bateson has called meristic—alterations in the number of parts in series. Such are in- crease or diminution in the number of the teeth, vertcbrcT, ribs, fingers, toes, etc. It is difficult to speak with j)rccision of internal organs, but probably some ca.ses at least of double kidneys and ureters, accessory ovaries and testicles, etc., come under this group. In addition there are what may be termed metabolic s])orts—conditions of albinism, ichthyo- sis, etc. It is common to class most of the examples of the first, meristic, class as instances of reversion; but the more one studies them the more evident it is that they must in general be included as .sports. They occur most often in those who ])rcsent no other signs of reversion, and, as al- ready stated, if nnmerical increase is an instance of reversion, numerical decrease cannot be; and vice versa. Supernumerary digits crop up in no *Dic Mutationsthenrie, 1901. '^The Dynamics of Living Matter, New York; MacMillan, 190G, 225. .3](https://iiif.wellcomecollection.org/image/b24907212_0001_0057.jp2/full/800%2C/0/default.jpg)
