PROGRESS IN THE STUDY OF VARIATION. 555 



First, it is evident that there is considerable misappre- 

 hension as to the sense in which the term discontinuity is 

 used in its appHcation to variation. Discontinuous varia- 

 tion is carelessly supposed by some to be synonymous with 

 "sport". While however a sport is a discontinuous varia- 

 tion, it is not in this restricted sense that the term should be 

 used. The word "sport" is generally used to denote a 

 variation connected with the normal by no intermediate 

 individuals. In many — perhaps in most — cases of discon- 

 tinuous variation there is a complete series of intergrading 

 individuals, not necessarily in each direct line of descent, but 

 occurring among the varying population. Discontinuity in 

 variation is a term of degree. A variation is discontinuous 

 in p7'oportion as intermediates between the variety and the 

 type are comparatively infrequent. For a variation is dis- 

 continuous if, when all the individuals of the population are 

 breeding freely together, there is not simple regression to 

 one mean form, but a sensible preponderance of the variety 

 over the intermediates. When such a population is seriated 

 in respect of the varying character for statistical study in 

 the manner with which naturalists have been familiarised 

 by the writings of Galton and others the curve of variation 

 has not one peak as in a monomorphic species, but has at 

 least two peaks. The essential feature of a discontinuous 

 variation is therefore that, be the cause what it may, there 

 is not complete blending between variety and type. The 

 variety persists and is not "swamped by intercrossing". 



While the word "sport" is expressly used to denote 

 something rare or monstrous, no such idea necessarily 

 attaches to discontinuous variation. When in a species 

 commonly known to us as a constant and comparatively 

 invariable form, an individual is found showing a striking 

 variation, we call it a sport. Thus the six-rayed Sarsia 

 ((2), p. 424) is a sport. But in some Medusae the presence 

 of individuals exhibiting similar meristic variations, either 

 regular or irregular, is so frequent that these "sports" are a 

 normal constituent in any sample.^ If in England a speci- 



^ For example Aiirelia, see Browne, Q. J. M. S., xxviii., p. 245 ; 

 various authors Nature, 1894; Zool. Cblt., ii. p. 37, etc., and Eucope, 

 Agassiz and Woodworth, Harv. Bull., Nov., 1896, p. 121. 



