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15

This also appears to be characteristic of the mountain forms of some species of Aldono gyrus¹). The fact that in Poland we find an increase in size associated with a denser pigmen-tation in Gyrinus as was also found in the nearly related Ge-nus Aidonogyrus in the high mountains of Abyssinia suggests that this is due to Iow temperaturę. In Dyłiscidae there is, likewise, a marked tendency for individuais to be morę darkly pigmented in the North. In Abyssinia the sex ratio also appe-ared to change with the altitude; there being a preponderance of females at high altitudes and of males at. Iow. No correspon-> increase in the proportion of females was noticeable in

ding

O

Poland indeed in all the species of which sufficient materiał was available males were in the majority.

The materiał from Poland has thrown light upon the in-teresting problem of Species-pairs and Species-complexes²).

In a paper on the Coleoptera of Wicken Fen by CHARLES E. ToTTENHAM and myselfⁿ) the species-pairs G. marinus-thomsoni and G. caspius-paykulli were discussed and attention wćs directed to the species complex G. nałator-substriatus-sufi-ricni. The Polish materia! was rich in specimens of G. nałator and G. substriałus yarieties and has increased our knowledge of this peculiarly interesting species-cornplex.

It seem probable that in the coimse of evolution the germ plism of some species becomes unstable and under the influenc of physiological conditions, or of isolatiom yarieties are pnduced which in time become so we

11 defined that they are In Abyssinia such an umtablc species appears to be Dineulus gondaricus while in Eiropc the species complex G. natator-substriatus-suffriani is a /ery beautiful example of the formation of species. After

re.OL>ni

izcd

as species or

sub-species.

i

C

i o p c r s

) O in e r - C o o p e r, 1930. Report on the Gyrinidae. Mr, O m e r-

Abyhbinian Fresh Waters (H u g h Scott

investi^ation of the

Fxieclition). Proc, Zool, Soc. London. 1930. pp, 415—427.

-) O m e r - C o o p e r, 1931. Spccies-Pairs in Insects, Naturę. Vol. 127,

_P. :37.

9 O m e r - C o o p e r, 1931. On Gyrinus natcitor L. and G. substriatus Stoli. Ent. Mo, Mag. Vol. LXVII, pp. 195 —198,

^;) O m e r - C o oper and T o t t e n h a m. 1932. The Coleoptera of Witkeri Fen, Pt, III, The Natural History of Wicken Fen, Pt, VI pp. 489—500,