| | | |2. Rocky Mountains. |Transition to Neotropical.| | |3. Alleghanies (or East U.S.)| | | |4. Canada. |Transition to Palæarctic. | +--------------+-----------------------------+--------------------------+ {83}CHAPTER V. CLASSIFICATION AS AFFECTING THE STUDY OF GEOGRAPHICAL DISTRIBUTION. A little consideration will convince us, that no inquiry into the causes and laws which determine the geographical distribution of animals or plants can lead to satisfactory results, unless we have a tolerably accurate knowledge of the affinities of the several species, genera, and families to each other; in other words, we require a natural classification to work upon. Let us, for example, take three animals--_a_, _b_, and _c_--which have a general external resemblance to each other, and are usually considered to be really allied; and let us suppose that _a_ and _b_ inhabit the same or adjacent districts, while c is found far away on the other side of the globe, with no animals at all resembling it in any of the intervening countries. We should here have a difficult problem to solve; for we should have to show that the general laws by which we account for the main features of distribution, will explain this exceptional case. But now, suppose some comparative anatomist takes these animals in hand, and finds that the resemblance of _c_ to _a_ and _b_ is only superficial, while their internal structure exhibits marked and important differences; and that _c_ really belongs to another group of animals, _d_, which inhabits the very region in which _c_ was found--and we should no longer have anything to explain. This is no imaginary case. Up to a very few years ago a curious Mexican animal, _Bassaris astuta_, was almost always classed in the civet family (Viverridæ), a group entirely {84}confined to Africa and Asia; but it has now been conclusively shown by Professor Flower that its real affinities are with the racoons (Procyonidæ), a group confined to North and South America. In another case, however, an equally careful examination shows, that an animal peculiar to the Himalayas (_Ælurus fulgens_) has its nearest ally in the _Cercoleptes_ of South America. Here, therefore, the geographical difficulty really exists, and any satisfactory theory of the causes that have led to the existing distribution of living things, must be able to account, more or less definitely, for this and other anomalies. From these cases it will be evident, that if any class or order of animals is very imperfectly known and its classification altogether artificial, it is useless to attempt to account for the anomalies its distribution may present; since those anomalies may be, to a great extent, due to false notions as to the affinities of its component species.