ges of climate to have taken place at all (compare pp. 75-80). We
really have no idea under what precise climatic conditions the Siberian
mammals lived in their original home. The only thing we can be certain
of is that the smaller burrowing mammals would not have chosen a wood
to live in, if they could possibly help it. Prairies, or sand-dunes with
short grass or shrubs, such as abound in Europe near the sea-coast,
would suit these species perfectly. If we suppose Northern Germany to
have been covered by sea (p. 156) during part of the Pleistocene Epoch,
forests would probably not have grown there for a very considerable time
afterwards, owing to the excessive salinity of the soil, but a tract of
sandy country would have been left on the retreat of the sea. Possibly a
slight change of climate in the original home of these steppe-species
may have reduced their habitable area, and thus caused their migration
into Europe.

But this migration problem cannot be solved without tracing the mammals
to their place of origin and investigating their early history. This I
shall attempt to do presently; meanwhile, it would be interesting to
note whether other groups of animals support Professor Nehring's
steppe-theory.

Among groups other than mammals, the most important, for the purpose of
drawing conclusions as to former physical conditions and climate, are
the mollusca. Their remains have been well preserved, and are easily
identified. Though Professor Nehring argues that the molluscs found
along with the small mammals harmonise perfectly with the assumption of
a steppe-climate (p. 212), I cannot at all agree with him. He enumerates
the following sixteen species as having been discovered by him:--

1. Pupa muscorum.
2. Chondrula tridens.
3. Cionella lubrica.
4. Patula ruderata.
5. Do. rotundata.
6. Helix striata.
7. Do. hispidia.
8. Do. tenuilabris.
9. Helix pulchella.
10. Do. hortensis.
11. Do. obvoluta.
12. Hyalinia radiatula.
13. Succinea oblonga.
14. Limnaea peregra.
15. Clausilia sp.
16. Pisidium pusillum.

Only two of these can be looked upon as typically northern species,
viz., _Patula ruderata_ and _Helix tenuilabris_, though both of them are
still found living locally in Germany. Some of the others are decidedly
southern species, like _Chondrula tridens_, _Helix obvoluta_, _H.
rotundata_, and _H. striata_. All the rest live and flourish, for
example, in I