THE INTRODUCTION OF NEW SPECIES. 7 



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 



