THE SPECIES CONCEPTION. XXV11 



The species of the mustelina-decora group, and those of the 

 apexfulva-leucorraphe group (see p. 275 and diagram on 

 p. 278) are certainly very closely related while A. lor at a 

 stands isolated. In Partulina most forms admitted as species 

 seem to belong to the first grade. 



Species and subspecies of all grades are usually composed 

 of numerous color-forms of Grade III that is, elementary 

 patterns, which may rarely occur in pure colonies, but almost 

 always in hybrid colonies comprising two or more patterns, 

 in an almost endless variety of combinations. 



What may be considered "fluctuating variations", that is, 

 variations in the potency with which a pattern may be ex- 

 pressed, among individuals of a pure colony, are of great 

 amplitude in Achatinellidce. Thus, plate 60, figs. 17 to 17c 

 may be presumed to be individuals having the same gametic 

 constitution, but differing in the degree of potency of the 

 factor controlling the deposition of pigment. Other examples 

 are pi. 56, figs. 14 to Ud ; 13 to 136 ; figs. 1 to Ic. Most of 

 the pure colonies show such variation in greater or less degree. 

 Most shells having highly developed color-patterns show 

 similar fluctuation. Cf. Neritina, Liguus, etc. 



In a hybrid colony there is often almost perfect segregation 

 of the color-patterns composing it, as in pi. 39, figs. 8 to 8e. 

 This is usually the case where only two or three patterns are 

 involved. Where the number is greater, the combinations be- 

 come numerous. We are getting into the realm of trihybrids 

 and multihybrids. Although we see only the phenotypes of 

 the gigantic Mendelian experiment, it is our belief that with 

 a good knowledge of the forms of any limited region, a trust- 

 worthy estimate of the elementary patterns present in a colony 

 not too complex may be formed, though of course it would be 

 absurd to say what the genotypic constitution of any individ- 

 ual of the colony might be. 



The good segregation which prevails in hybrid colonies 

 leads collectors to assort their shells. The strikingly diverse 

 patterns were not unnaturally described as distinct species 

 by European conchologists, who were ignorant of the facts of 

 association. Moreover, before the Mendelian era, it seemed 

 improbable that such diverse patterns could be hybrids. 



