INTRODUCTION Ixvii 



mileage is small, would, but for some chance, remain isolated for ages, if not for 

 ever. How stationary some of these land-shells are, I had many chances of observing, 

 for in some cases I had the same individual under observation for weeks together. 

 A white variety of Achatinella redfieldi, which I had occasion to pass by almost daily 

 for many weeks, was always seen at rest just below the fork of a large branch of a 

 lichen-covered Ohia tree. Though there were many showery or wet days, it was 

 never absent from this spot by day, though it may, of course, have moved at night 

 and returned. This was a healthy animal, the shell not at all worn by age. A small 

 dead lichen-covered tree, supporting individuals of A. theodorei in 1893, ^'^-S still 

 occupied by these or their descendants three years later, the adjoining bushes being 

 unoccupied. These shells had the appearance of being a stunted, depauperated form 

 of A. proxvna, produced by isolation on a wind-swept ridge with its stunted vegetation. 

 In 1902 I looked in vain for this particular colony. To causes, as here given, is due 

 the multiplicity of species or local forms of the Achatinellidae. 



Probably few insects are so restricted in range, or so easily kept isolated, as are 

 many of the Achatinellidae, for many of the most sluggish insects are, I think, likely 

 to be more frequently distributed by accidental means than the Mollusca. Still in 

 the insects there are cases known both of distinct species of similar habits and having 

 the same food, which merely occupy a different area of the same island, and of a single 

 species, which, occupying different areas, exhibits different variation in these two areas 

 — or in fact is on the way to form two. 



Very often, however, we find species, extremely closely allied, occurring habitually 

 in the same locality and not geographically isolated. Thus, even within a few 

 square yards, the three species of Longicorn beetles, Plagithinysus darivinianus, 

 P. lamarckianus and P. varians occur. It is hardly conceivable that species can 

 be more closely allied than these and remain distinct. Though so similar, the 

 species keep quite apart. Each keeps to its own food-plant, and though occurring 

 on adjoining trees the species do not mix nor interbreed. P. darwinianus has been 

 found chiefly on Sophora, lamarckianus on Pipturus, varians on Acacia. We have 

 observed great numbers of all these species in the field, but have never found even 

 a stray specimen of one frequenting the tree affected by another or in company with 

 it, even though these trees grew side by side. Thus these three species, though 

 not geographically isolated, are isolated by their habits. I know no insects that 

 could be more profitably experimented with by breeding in the field than these and 

 other species of Plagithmysus. 



It is certainly quite e.xceptional for a species of these beetles to forsake its 

 proper food-plant and attach itself to quite a different tree, as is evident from the 

 fact that a species in most cases has been found on one kind of tree and only one. 

 It would be of great interest to know whether individuals, bred for the first time 

 on a tree other than the normal food-plant, remain attached to that tree, or wander 



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