- [Footnote: fish, 700 reptiles, 4000 birds, and 500 mammalia. Europe
has about 80 species of indigenous mammalia, 400 birds, and 30 reptiles. In the Northern temperate zone, therefore, the species of birds are five times more numerous than those of mammalia, as there are in Europe five times as many Compositæ as there are Amentaceæ and Coniferæ, and five times as many Leguminosæ as there are Orchideæ and Euphorbiaceæ. In the southern hemisphere the ratio of mammalia is in tolerably striking agreement, being as 1 to 4·3. Birds, and still more reptiles, increase in the number of species in approaching the torrid zone more than the mammalia. Cuvier's researches might lead us to believe that the proportion was different in the earlier state of things, and that many more mammalia had perished by revolutions of Nature than birds. Latreille has shewn what groups of insects increase towards the pole, and what towards the equator. Illiger has given the countries of 3800 species of birds according to the quarters of the globe: it would have been much more instructive if the same thing had been done according to zones. We should find little difficulty in comprehending how on a given space of the earth's surface the individuals of a class of plants or animals limit each other's numbers, or how, after long continued contest and many fluctuations caused by the requirements of nourishment and mode of life, a state of equilibrium should be at last established; but the causes which have limited not the number of individuals of a form, but the forms themselves, in a particular space, and founded their typical diversity, are placed beneath the impenetrable veil which still conceals]*
