8 NATURAL SELECTION I 
systems of classification which arrange species or groups in 
circles, as well as those which fix a definite number for the 
divisions of each group. The latter class have been very 
generally rejected by naturalists, as contrary to nature, 
notwithstanding the ability with which they have been 
advocated ; but the circular system of affinities seems to have 
obtained a deeper hold, many eminent naturalists having to 
some extent adopted it. We have, however, never been able 
to find a case in which the circle has been closed by a direct 
and close affinity. In most cases a palpable analogy has been 
substituted, in others the affinity is very obscure or altogether 
doubtful. The complicated branching of the lines of affinities 
in extensive groups must also afford great facilities for giving 
a show of probability to any such purely artificial arrange- 
ments. Their death-blow was given by the admirable paper 
of the lamented Mr. Strickland, published in the Annals of 
Natural History, in which he so clearly showed the true 
synthetical method of discovering the Natural System. 
Geographical Distribution of Organisms 
If we now consider the geographical distribution of animals 
and plants upon the earth, we shall find all the facts beautifully 
in accordance with, and readily explained by, the present 
hypothesis. A country having species, genera, and whole 
families peculiar to it, will be the necessary result of its 
having been isolated for a long period, sufficient for many 
series of species to have been created on the type of pre- 
existing ones, which, as well as many of the earlier-formed 
species, have become extinct, and thus made the groups 
appear isolated. If in any case the antitype had an extensive 
range, two or more groups of species might have been formed, 
each varying from it in a different manner, and thus producing 
several representative or analogous groups. The Sylviade of 
Europe and the Sylvicolide of North America, the Heliconide 
of South America and the Euplceas of the East, the group of 
Trogons inhabiting Asia and that peculiar to South America, 
are examples that may be accounted for in this manner. 
Such phenomena as are exhibited by the Galapagos Islands, 
which contain little groups of plants and animals peculiar to 
themselves, but most nearly allied to those of South America, 
