According to the laws and causes of distribution discussed in the preceding chapters, we should find limited and defined distribution to be the rule, universal or indefinite distribution to be the exception, in every natural group corresponding to what are usually regarded as families and genera; and so much is this the case in nature, that when we find a group of this nominal rank scattered as it were at random over the earth, we have a strong presumption that it is not natural; but is, to a considerable extent, a haphazard collection of species. Of course this reasoning will only apply, in cases where there are no unusual means of dispersal, nor any exceptional causes which might determine a scattered distribution. From the considerations now adduced it becomes evident, that it is of the first importance for the success of our inquiry to secure a natural classification of animals, especially as regards the families and genera. The higher groups, such as classes and orders, are of less importance for our purpose; because they are almost always widely and often universally distributed, except those which are so small as to be evidently the nearly extinct representatives of a once more extensive series of forms. We now proceed to explain the classification to be adopted, as low down as the series of families. To these, equivalent English {85}names are given wherever they exist, in order that readers possessing no technical knowledge, may form some conception of the meaning of the term "family" in zoology. The primary divisions of the animal kingdom according to two eminent modern authorities are as follows: HUXLEY. CARUS AND GERSTAEKER. Classification of Animals Handbuch der Zoologie (1869). (1868). 1. Protozoa } } 1. Protozoa. 2. Infusoria } 3. Coelenterata 2. Coelenterata. 4. Annuloida 3. Echinodermata. { 4. Vermes. 5. Annulosa { { 5. Arthropoda. 6. Molluscoida 6. Molluscoida. 7. Mollusca 7. Mollusca. 8. Vertebrata 8. Vertebrata. For reasons already stated it is only with the fifth, seventh, and eighth of these groups that the present work proposes to deal; and even with the fifth and seventh only partially and in the most general way. The classes of the vertebrata, according to both the authors above quoted, are: 1. Mammalia. 2. Aves. 3. Reptilia. 4. Amphibia. 5. Pisces, in which order they will be taken here. The sub-classes and orders of mammalia are as follows: MAMMALIA.