for homologous forms. This
difficulty applies equally to living groups, and it is so important that
a few instances may not be amiss.

There is undeniable similarity between the faunas of Madagascar and
South America. This was supported by the Centetidae and Dendrobatidae,
two entire "families," as also by other facts. The value of the
Insectivores, Solenodon in Cuba, Centetes in Madagascar, has been much
lessened by their recognition as an extremely ancient group and as a
case of convergence, but if they are no longer put into the same
family, this amendment is really to a great extent due to their widely
discontinuous distribution. The only systematic difference of the
Dendrobatidae from the Ranidae is the absence of teeth, morphologically
a very unimportant character, and it is now agreed, on the strength of
their distribution, that these little arboreal, conspicuously coloured
frogs, Dendrobates in South America, Mantella in Madagascar, do not form
a natural group, although a third genus, Cardioglossa in West Africa,
seems also to belong to them. If these creatures lived all on the
same continent, we should unhesitatingly look upon them as forming a
well-defined, natural little group. On the other hand the Aglossa, with
their three very divergent genera, namely Pipa in South America, Xenopus
and Hymenochirus in Africa, are so well characterised as one ancient
group that we use their distribution unhesitatingly as a hint of a
former connection between the two continents. We are indeed arguing in
vicious circles. The Ratitae as such are absolutely worthless since they
are a most heterogeneous assembly, and there are untold groups, of
the artificiality of which many a zoo-geographer had not the slightest
suspicion when he took his statistical material, the genera and
families, from some systematic catalogues or similar lists. A lamentable
instance is that of certain flightless Rails, recently extinct or
sub-fossil, on the isalnds of Mauritius, Rodriguez and Chatham. Being
flightless they have been used in support of a former huge Antarctic
continent, instead of ruling them out of court as Rails which, each in
its island, have lost the power of flight, a process which must have
taken place so recently that it is difficult, upon morphological
grounds, to justify their separation into Aphanapteryx in Mauritius,
Erythromachus in Rodriguez and Diaphorapteryx on Chatham Island.
Morphologically they may well form but one gen