XXX THE SPECIES CONCEPTION. 



for groups of individuals having moderately coherent com- 

 binations of characters, and yet not properly to be ranked 

 as subspecies. In this sense we have used such terms 

 as ' * olivacea pattern " ; ' dunkeri pattern. ' In other cases 

 we have avoided naming the forms, using numbers in- 

 stead. The ideal (if impracticable) method would be to have 

 a name or symbol for each unit character, and express the 

 several combinations like a chemical formula; but although 

 almost all the world is breeding animals or plants at present, 

 it is not likely that we will ever have enough knowledge of 

 unit characters to apply this method generally in nomen- 

 clature ! 



Color-pattern is usually independent of direction of coil of 

 the shell, but in some cases, noted under various species, these 

 characters are linked in inheritance. See under Partulina 

 virgulata, p. 358, etc. 



The facts indicate that mutations may appear anywhere 

 in the area of a species, and spread from the initial point as 

 far as time and conditions permit, mingling with the pre- 

 existing forms of the species. The population is thus becom- 

 ing constantly more complex. It occasionally happens that 

 one comes upon what appears to be a mutation in an early 

 stage of its career. Thus, near the foot of a small ridge in 

 Popouwela, there is a large and prolific colony of Acha- 

 tinella mustelina (pi. 63, figs. 10 to lOc). In the midst of 

 this colony, in an area of two or three rods square, we found 

 20 or 30 pure white individuals, most of them immature. Not 

 one was found in other parts of the colony, which three of 

 us looked over pretty thoroughly, or in any other colony in 

 the same district. The inference seems fair that the albino 

 form had very recently arisen in that place. 



Many cases of special color-forms known from single or 

 very few trees are recorded in the descriptive part of this 

 work. Some of these are doubtless remains of races once 

 more widely spread, especially when they occur in decadent 

 forest; but others we can hardly doubt, are new mutations. 

 The collections made to-day very often show color-forms dif- 

 fering from those of the same valleys made sixty years ago; 



