I THE INTRODUCTION OF NEW SPECIES 7 
common antitype, then the series of affinities will be com- 
pound, and can only be represented by a forked or many- 
branched line. Now, all attempts at a Natural classification 
and arrangement of organic beings show that both these plans 
have obtained in creation. Sometimes the series of affinities 
can be well represented for a space by a direct progression 
from species to species or from group to group, but it is 
generally found impossible so to continue. There constantly 
occur two or more modifications of an organ or modifications 
of two distinct organs, leading us on to two distinct series of 
species, which at length differ so much from each other as to 
form distinct genera or families. These are the parallel series 
or representative groups of naturalists, and they often occur 
in different countries, or are found fossil in different forma- 
tions. They are said to have an analogy to each other when 
they are so far removed from their common antitype as to 
differ in many important points of structure, while they still 
preserve a family resemblance. We thus see how difficult it 
is to determine in every case whether a given relation is an 
analogy or an affinity, for it is evident that as we go back 
along the parallel or divergent series, towards the common 
antitype, the analogy which existed between the two groups 
becomes an affinity. We are also made aware of the diffi- 
culty of arriving at a true classification, even in a small and 
perfect group; in the actual state of nature it is almost 
impossible, the species being so numerous and the modifica- 
tions of form and structure so varied, arising probably from 
the immense number of species which have served as anti- 
types for the existing species, and thus produced a compli- 
cated branching of the lines of affinity, as intricate as the 
twigs of a gnarled oak or the vascular system of the human 
body. Again, if we consider that we have only fragments of 
this vast system, the stem and main branches being repre- 
sented by extinct species of which we have no knowledge, 
while a vast mass of limbs and boughs and minute twigs and 
scattered leaves is what we have to place in order, so as to 
determine the true position which each originally occupied 
with regard to the others, the whole difficulty of the true 
Natural System of classification becomes apparent to us. 
We shall thus find ourselves obliged to reject all those 
