THE INTRODUCTION OF NEW SPECIES. q 
of organic beings show, that both these plans have 
obtained in creation. Sometimes the series of affini- 
ties 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 con- 
tinue. There constantly occur two or more modifica- 
tions 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 formations. 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 diver- 
gent series, towards the common antitype, the analogy 
which existed between the two groups becomes an 
affinity. We are also made aware of the difficulty 
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 modifications of form and structure so varied, 
arising probably from the immense number of species 
which have served as antitypes for the existing spe- 
cies, and thus produced a complicated branching of 
