July 18, 1872J 



NATURE 



223 



Number of Species and Varielics on Oaliu, and the 

 Rej^ions they Inhabit 



The number of species of Achatinellina? on Oahu may 

 be estimated at about 1S5, representing eight genera, 

 but belonging chielly to five. Besides these there are 

 many small HtliCcS, which would probably bring the 

 whole number of the species of Helicida; on Oahu \ip to 

 about 200, the varieties numbering Soo or 903. None of 

 these species— excepting, perhaps, one or two of the 

 ground species, which are reported to have been found on 

 the island of Maui — exist anywhere beyond the narrow 

 circuit of this island, the extreme length of which is 

 only 6o miles, with an average breadth of about 15 miles. 

 Nor does any one species occupy a large proportion of 

 c\en this area. Nearly all arc confined to the forest 

 regions skirting two ranges of mountains. The chief 

 range, on the north-cast side of the island, is about 40 

 miles in length. The forest region that covers it has an 

 average breadth of five or six miles. The range on the 

 opposite side of the island is about half as long, and has 

 only about one-fourth as much forest land. The north- 

 eastern side of the island, owing to its receiving the trade- 

 winds when they first strike the island, enjoys a moister 

 climate and possesses more luxuriant vegetation than the 

 south-western side. We are therefore somewhat prepared 

 to find that seven-eighths of the species, or about 175, are 

 found in the former area, about 40 miles in length and 

 five or six miles in width. Passing over innumerable 

 minor variations, the varieties found in this area are no 

 less than 700 or Soo. 



That so large a number of species and varieties of land 

 molluscs should be found within so small an area is, I 

 think, unparalleled in the records of conchology ; but that 

 this great number of forms should, with but two or three 

 exceptions, be found nowhere beyond these narrow limits, 

 not even on the other half of the same island, is still more 

 astonishing. What shall we say when we discover that 

 no one of these species is distributed over even half of 

 this small mountain range ; but that in most cases they 

 are restricted to areas of from one to five miles in length ? 

 Have we found one of the " centres of creation " ? 



The principal facts in the geographical distribution of 

 these forms are as follows : — 



I. — Facts relating to the Position and Natural Dh'isions 

 of the Territory 



The Sandwich Islands are surrounded by a wider ex- 

 panse of open ocean than any other islands of equal 

 extent. The forms of Helicida: on this group differ widely 

 from those of other lands. Not only do we find distinct 

 species and genera, but a separate group of genera. 



The group of islands may be divided into four pro- 

 vinces, each of which has a separate set of species and 

 possesses one genus or more that is peculiar to the pro- 

 vince, besides other genera that are common to several 

 provinces, (a) On Kauai alone is found the Caniia ; 

 \b) on Oahu, the Buliniclla s.ndHiliiteriilaj- (e) on Maui, 

 Molokai and Lanai, the Neivcoinbia (Pfeiffer) ; [d) on 

 Hawaii, certain peculiar forms that have not yet been fully 

 collected or classified. Kauai, which is separated from the 

 olher islandsby the widest channel, has the forms that dit'fer 

 the most widely from those of the central part of the group. 



Most of the species are confined to the forests of moun- 

 tain regions ; and where, as on Oahu and JVIaui, there are 

 two regions of forest divided by several miles of grass 

 country, the island is also divided into two sections, 

 having but few, if any, species in common. 



On the island of Oahu, the two sections which occupy 

 separate mountain ranges are divided into many minor 

 sections in the following manner. From each side of the 

 main range project mountain ridges, which separate deep 

 valleys a mile or two in width. Each of these valleys is a 

 subordinate section, having its own varieties and in many 

 instances its own species, which are found nowhere else. 



1 1. — Facts relating to the Variation and Affinity of Species 



Nearly all the species of one genus found on one moun- 

 tain range are connected by varieties presenting very 

 minute gradations of form and colour. Species of the 

 same genus on different islands are not so completely 

 connected by intermediate forms. 



The degree of difference between several species of the 

 same group is in proportion to their separation in space. 



Nearly allied species, occupying neighbouring localities, 

 pass into each other by all the intermediate gradations of 

 form and colour, while those whose homes are separated 

 by a distance of eight or ten miles, cannot be connected 

 by minute gradations without bringing in some of the 

 forms occupying the intermediate territory. 



As the relations to each other of the valleys surrounding 

 any mountain are determined by the shape of the moun- 

 tain with its ridges, so are the relations of these species to 

 each other, in the arrangement of their affinities and diver- 

 gences, influenced by the same cause. As the geometrical 

 relations to each other of valleys clustered around one 

 central peak differ from those distributed on either side of 

 a long range, so do the affinities and divergences (the 

 structural relations to each other) of the species on one 

 of the high solitary mountains of Maui differ from those on 

 one of the mountain ranges of Oahu. On the eastern 

 range of Oahu the species of Achatinella are distributed 

 on both sides of the mountain in parallel lines, the 

 extremes of divergence being in the forms at the ends of 

 the range. But either on East Maui or West Alaui, 

 where the arrangement of the valleys is more concentric, 

 the varieties of any one group of species converge so 

 rapidly toward one central type, that it is difficult to distri- 

 bute them into well-defined species. 

 1 1 1 . — Facts relating to the comparative Area occupied by 

 Species of different Classes 



Th^ average length of the area occupied by different 

 species is perhaps five miles. 



Field species have the widest range. Arboreal species 

 have the narrowest range. Ground species found in forest 

 regions have a medium range. 



Many interesting questions are suggested by these facts. 

 How can we account for the species being restricted in 

 their distribution to such narrow limits .'' Why do not 

 the species of North-eastern Oahu pass over their narrow 

 bounds and become mingled throughout the whole extent 

 of that short mountain range 1 



Questions Suggested 



The minute gradations by which the species of each 

 genus of the Achatinellinx are connected with each other 

 strongly favours the belief that many of them must have 

 been derived the one from the other by successive varia- 

 tions. If created independently, why should there be 

 such gradation .? Why should the species of one group 

 be so arranged that those intermediate in form are found 

 in intermediate localities.' It maybe said that those of 

 one group, which are gradated together by intermediate 

 varieties, are not only from one stock, but are one species. 

 If they are one species, how shall we account for the 

 difference of size and form, the entire change of colour, 

 and in many instances of habits, leading some of the so- 

 called varieties to avoid plants that are chosen by other 

 varieties living only a few miles distant, and to choose 

 plants that are rejected by the others ? Why should tlie 

 Achatinella- feed on Kukui trees [Aleurites triloba) in the 

 eastern districts of the island, and in the north-western 

 choose small shrubs, leaving the Kukui trees to the Buti- 

 nielhc ? Whether we call the different forms species or 

 varieties, the same questions are suggested, as to how 

 they have arisen, and how they have been distributed in 

 their several localities. 



In attempting to answer these questions, we find it 

 difficult to point to any of those active causes of accumu- 

 lated variation, classed by Darwin as illustrations of 



