218 Natural science. April, 



measurable characteristics of plants and animals," has not yet been 

 published ; but, as it formed the subject of the discussion which he 

 introduced at the meeting referred to in our last paragraph, it is 

 already a subject for comment. It may be taken for granted that 

 most biologists agree that variations of a nature similar to those now 

 occurring among plants and animals have been the source of the 

 differences between the varieties, species, genera, and so forth in which 

 existing animals and plants are classified. Now there are two 

 great classes of variations. There are large and striking variations, 

 sometimes called sports, or monstrosities when they are more extreme 

 and possibly detrimental to the organisms in question. Then there 

 are numerous, almost innumerable, small variations that occur in 

 every part of the structure and in every physiological quality of the 

 animal. Have species come from the large or from the small 

 variations ? 



Mr. Bateson, in his " Materials for the Study of Variation," 

 called the former set instances of discontinuous variation, the 

 latter set instances of continuous variation. He, as many others 

 before him, suggested that, as species generally are discontinuous, it 

 is more than possible that discontinuous variations have been the 

 chief source of varieties and of species. The chief difficulty in the 

 other view is that it is impossible to see in many cases, perhaps 

 in any case, how small variations could have a value in selection, 

 how, in fact, the gradual increments in a continuous change could 

 each have had a determining value in the selection of the animal or 

 plant. 



The Royal Society's committee, or rather Professor Weldon for 

 it, made a statistical inquiry into the variation of certain dimen- 

 sions in the shore-crab. In the case of one of the dimensions investi- 

 gated, the " frontal breadth " of the crab, it was found that among 

 seven thousand females practically all possible variations occurred, in 

 the proportions suggested by probability. Precisely the same was 

 the case when the variations in the dimension of the " right dentary 

 margin " were appropriately reduced to standard and tabulated. In 

 both cases, as Professor Weldon's curves showed to a remarkable 

 and convincing degree, the number of individuals in which each 

 variation occurred was such a number as mathematical probability 

 would suggest. In other words, the occurrence of each variation is 

 as much the result of " chance " as the particular combination that is 

 made when a dozen dice are thrown on the table. Any single throw 

 is " chance " ; but if a large enough number of throws be made, each 

 combination turns up the number of times that the mathematical 

 possibility determines. Although Professor Weldon did not say so, 

 it must have occurred to many listeners that this first result of 

 statistical inquiry on variation was in direct contradiction to those 

 who assert that variation is not a matter of "chance," but that it 

 occurs in determined directions. 



