126 PROBLEMS OF GENETICS 



do well to make himself acquainted with the original from 

 which the following notes are taken. He speaks for example 

 of Helix lapicida. This is on the whole a constant form ranging 

 up to the altitude of 1,300 m., common all over France except 

 at great heights and in the Olive regions where it is restricted 

 to moist places. Though subjected to such diverse conditions 

 it shows only trivial variations in colour and other respects 

 throughout its distribution, excepting that on both sides of the 

 Pyrenees it has a very distinct sporadic variety called Andorrica 

 or microporus. This variety occurs here and there, together 

 with the type-form sometimes in colonies (pp. 26-30 and 86). 

 Bulimus detritus though more restricted in geographical range 

 is a much more variable form. It exhibits great variations in 

 colour, form, and size, and as Coutagne well insists, these are 

 independent of each other. Foreshadowing the methods of 

 factorial analysis he suggests that distinctions in each respect, 

 the "modes" as he calls them, should be denoted by a letter, 

 or if desired, by a name, and the several combinations of differ, 

 ences might thus be most logically and usefully expressed. Of 

 such combinations he says there are at least 18, all of which can 

 be found. The whole possible series does not necessarily occur 

 in the same place, and various localities are characterised by 

 the presence or absence of certain of the combinations as Cou- 

 tagne calls them, and by the relative frequency with which they 

 occur. The ideas thus enunciated are much in advance of the 

 ordinary practice of systematists, who give names to forms which 

 are nothing but accidental combinations of factors, just as the 

 horticulturists for practical reasons give names to similar com- 

 binations, which as we now know are merely specially noticeable 

 terms in a long series of possibilities. In each case it is rather 

 the factors which should be named than the forms which are 

 constituted by their casual collocation. In this special example 

 of Bulimus detritus the 18 forms are made by the combinations 

 of three pairs of independent factors. Besides these combina- 

 tions which may occur anywhere or almost anywhere in the dis- 

 tribution there are two more distinct local forms, each of which 

 is regarded by Coutagne as probably constituting a fresh " mode," 

 perhaps compatible with the others. 



