54 MISCELLANEOUS EXAMPLES. [introd. 



occurred. No marked variation in the upper-sides was to be seen. 

 These specimens were all from the Khasia hills, Assam, but there 

 was of course no evidence that all were flying together. 



One of the most interesting examples of discontinuous Variation in 

 colour-patterns is the case of ocellar markings or eye-spots. Upon 

 this subject nothing need here be said as the evidence will be given in 

 detail in the course of this volume (see Chap. xiii.). 



SECTION X. 



Discontinuity in Substantive Variation. — Miscellaneous 



Examples. 



Of the discontinuous occurrence of Substantive Variation, the 

 manifestations are many and diverse. CWe have seen that in such 

 features as size, colour, and colour-patterns, Variations may be 

 discontinuous, and a form may thus result, differing markedly 

 from the type which begot it/ Variation in the proportions or 

 the constitution of essential parts may no less suddenly occur. 

 The range of these phenomena is a large one, but for the purposes 

 of this Introduction a few examples must suffice in general 

 illustration of their scope. 



A discontinuous variation which is familiar to all is that of 

 " reversed " varieties, especially of Molluscs and Flat-fishes. Such 

 varieties are formed as optical images of the body of the type. In 

 both of the groups named, some species are normally right-handed, 

 others being normally left-handed, while as individual variations 

 reversed examples are found. In Molluscs this is not peculiar to 

 Gasteropods with spiral shells, but may occur also both in Lima- 

 cidae (slugs) 1 and in Lamellibranchs' 2 . Such variation is commonly 

 discontinuous, and the two conditions are alternative. The fact 

 that the reversed condition may become a character of an estab- 

 lished race is familiar in the case of Fvsiis antiquus. This shell 

 is found in abundance as a fossil of the Norwich Crag, such 

 specimens being normally left-handed, though the same species at 

 the present day is a right-handed one. Of the left-handed form a 

 colony was discovered by MacAndeew on the rocks in Vigo Bay 3 . 

 It was there associated with certain other shells proper to the 

 Norwich Crag. This discovery seemed to Edward Forbes to be so 

 remarkable that he looked on it as corroborative evidence of a 

 special connexion between the fauna of Vigo Bay and the Crag 

 fossils 3 . Jeffreys had the same variety from Sicily 4 . 



1 For example, a sinistral Avion, Baudon, Jour, de Conch., xxxn. 1884, p. 320, 

 and many others. 



2 Sinistral Tellina, Fischeh, P., Jour, de Conch., xxviii. 1880, p. 234. The 

 same is recorded in several other genera. 



3 Seven specimens, Ann. N. H., 1849, p. 507. 



4 Brit. Conch., i. p. 326. 



