24
T. B. REYNOLDSON
Of these (b) is the least likely on present-day evidence of the extent,
duration, condition and position of land bridges between Ireland and
the mainland. Corbet favoured (c) to account for the Lusitanian fauna.
Water for drinking purposes must have been transported often and in
quantity by travellers, and triclads, especially the spring-dwelling species, may have been spread to some extent in this way. Fish stocking
has undoubtedly contributed also in recent times and probably earlier.
Bilge water of ships may be concerned in the dispersal of some aquatic
animals (Williams, 1962b) but is unlikely to have been of importance for
such delicate organisms as triclads. However, the widespread distribution and large number of species in Ireland argues forcibly against introduction in these accidental ways particularly in view of their slow dispersal on the mainland and the rarity of several species here, although
it is appreciated that they may not always have been ra.re. An important
argument against this method of introduction is the fact that both
P. nigra and P. felina have not succeeded in reaching Sweden, although
both occur nearby on the mainland of Denmark and in Germany
(Thienemann, 1950): the commerce across this narrow sea must have
always been very heavy. This hypothesis also fails to account for the
restricted fauna of Anglesey which lies on one of the trade routes between the mainland and Ireland. Anglesey also furnishes a very convincing demonstration of the ineffectiveness of birds in spreading triclads, since the island is separated from the mainland, where all four
species occur, by a strip of seawater only some 300-400 m wide, a separation which has existed since early post-glacial times (Greenly, 1919).
Only two lake species, P. nigra which is widespread and P. tenuis found
in two lakes, have succeeded in crossing the narrow barrier; the former
probably by its own movements before separation, the latter via troutstocking. Furthermore, if this type of introduction is important, Dugesia
gonocephala should have been able to cross the Channel and establish
itself in Brit,ain, since as Ullyott (1936) pointed out, the climatic and
ecological setting i s similar on both sides. But this species has not been
confirmed in Britain (Dahm, 1955; Reynoldson, 1956~). I n contrast it
might be mentioned that Dugesia tigrina (Girard) has succeeded in
establishing itself in some half-dozen places at least in Britain, probably
as the result of trade in freshwater fish for aquaria between Britain and
the Americas. However, if a narrow belt of seawater such as the Menai
Straits between the Welsh mainland and Anglesey is an effective barrier,
this raises the problem of how triclads have crossed the main watersheds. If not transported by the activities of birds or man, this leaves
their own movement as the most likely explanation. It may be that triclads have crossed the watersheds in the plains where continuity is more
likely and then dispersed up river. It is also possible that there have been
T. B. REYNOLDSON
Of these (b) is the least likely on present-day evidence of the extent,
duration, condition and position of land bridges between Ireland and
the mainland. Corbet favoured (c) to account for the Lusitanian fauna.
Water for drinking purposes must have been transported often and in
quantity by travellers, and triclads, especially the spring-dwelling species, may have been spread to some extent in this way. Fish stocking
has undoubtedly contributed also in recent times and probably earlier.
Bilge water of ships may be concerned in the dispersal of some aquatic
animals (Williams, 1962b) but is unlikely to have been of importance for
such delicate organisms as triclads. However, the widespread distribution and large number of species in Ireland argues forcibly against introduction in these accidental ways particularly in view of their slow dispersal on the mainland and the rarity of several species here, although
it is appreciated that they may not always have been ra.re. An important
argument against this method of introduction is the fact that both
P. nigra and P. felina have not succeeded in reaching Sweden, although
both occur nearby on the mainland of Denmark and in Germany
(Thienemann, 1950): the commerce across this narrow sea must have
always been very heavy. This hypothesis also fails to account for the
restricted fauna of Anglesey which lies on one of the trade routes between the mainland and Ireland. Anglesey also furnishes a very convincing demonstration of the ineffectiveness of birds in spreading triclads, since the island is separated from the mainland, where all four
species occur, by a strip of seawater only some 300-400 m wide, a separation which has existed since early post-glacial times (Greenly, 1919).
Only two lake species, P. nigra which is widespread and P. tenuis found
in two lakes, have succeeded in crossing the narrow barrier; the former
probably by its own movements before separation, the latter via troutstocking. Furthermore, if this type of introduction is important, Dugesia
gonocephala should have been able to cross the Channel and establish
itself in Brit,ain, since as Ullyott (1936) pointed out, the climatic and
ecological setting i s similar on both sides. But this species has not been
confirmed in Britain (Dahm, 1955; Reynoldson, 1956~). I n contrast it
might be mentioned that Dugesia tigrina (Girard) has succeeded in
establishing itself in some half-dozen places at least in Britain, probably
as the result of trade in freshwater fish for aquaria between Britain and
the Americas. However, if a narrow belt of seawater such as the Menai
Straits between the Welsh mainland and Anglesey is an effective barrier,
this raises the problem of how triclads have crossed the main watersheds. If not transported by the activities of birds or man, this leaves
their own movement as the most likely explanation. It may be that triclads have crossed the watersheds in the plains where continuity is more
likely and then dispersed up river. It is also possible that there have been