OCR Text |
Show 288 MR. F. P. BEDFORD ON MALAYAN ECHINODERMS. [Mar. 20, also at Singapore. The question of the extent to which the same species alters as it migrates from its specific-centre is one the solution of which must lead to a clearer idea of the mode of origin of new species (or rather perhaps new " form-units " ) . In many cases the same range of variation appears to occur throughout the whole area of distribution of tbe species, while in other cases we can speak of local varieties or local races; and in such, attempts have been made to trace a gradual progressive modification as we pass outwards from the supposed original " centre." This has been exemplified in some detail by Dr. Doderlein ' for two species of Culcita. In this genus the form and distribution of the pore-areas over the abactinal surface vary considerably within the limits of the same species, and in the two species, C. schmideliana and C. novce-guinece, these pore-areas have a similar independent range of variation from forms with small disconnected areola? to others in which the areolae merge into one another, leaving small isolated spaces devoid of pores. C. schmideliana var. ceglonica and C. novce-guinece var. plana belong to the former type, while C. schmideliana var. africana and C. novce-guinece var. arenosa belong to the latter type. The distribution of the two species is as follows : C. schmideliana occurs in the western part of the Indian Ocean, while C. novce-guinece extends throughout the Malay Archipelago and Eastern Pacific. Dr. Doderlein points out that at Ceylon var. ceglonica occurs, while at Sumatra and Java, the most western points to which C. novm-guinece extends, var. plana is found; and from this he draws the conclusion that " wo die Verbreitungsgrenze beider Arten aneinanderstosst, finden sich Varietaten, die einander auffallend ahneln." From this it would appear that the two species originated from a form intermediate between var. ceylonica and var. plana, and that the specific-centre of this form would lie between Ceylon and Malaysia. This, however, is at once falsified by finding the opposite extreme of variation, viz. var. arenosa, at Singapore, a point nearer to Ceylon than the places at which var. plana has been collected. This merely shows how important it is to determine the distribution of varieties before questions of position of specific-centre can be solved, and it will serve, I hope, as an apology for what may be regarded as unnecessary detail in the descriptions I have given of specimens collected. In the case of Astropecten javanicus we appear to have a case of progressive modification as we pass from Java through the Straits of Malacca to the Mergui Archipelago, where A. andersoni, which I believe to be a variety of A. javanicus, is found ; but it would be rash to assume this until the region is more fully worked out. I have adopted in all cases the generic and specific notation employed by Professor Ludwig 2, as it seems to me preferable to attempt to impress some uniform scheme into systematic reports rather than to wage an incessant war of words in defence of doubtful claims to priority. 1 Semon's Zool. Forsch. in Austr. &c. Bd. v. Lf. 3, 1896, pp. 310-316 Bronn's Thier-Eeich, Bd. ii. Abth. 3, Asteroidea, 1899. |