ORGANIC AGENCIES. 12? 



class (non-placentals), including kangaroos, opossums, 

 ornithorhynchus, etc., which, with the exception of a few 

 species of opossums, are found only in Australia and the 

 island appendages of that continent. Madagascar is sep- 

 arated by a deep sea from Africa, and we therefore find 

 the organic forms entirely different from those of the 

 neighboring continent, or of any other part of the world. 

 It is especially characterized by the great number of 

 lemurs. On the Galapagos (a small group of islands off 

 the western coast of South America, but separated by a 

 deep sea) the animals and plants are all peculiar. Eeptiles 

 of strange aspect abound, but no mammals (except, per- 

 haps, one species of mouse) are known. 



Thus we see that species are limited north and south 

 by temperature, and in every direction by physical bar- 

 riers. If, now, we add peculiar soil and climates (as in 

 Utah, Arizona, etc.), which, of course, control vegetation 

 and, therefore, animal life, it is easy to see that all these 

 limiting causes produce groups of species confined within 

 certain areas, and differing from other groups, sometimes 

 overlapping and sometimes trenchantly separated. 



Element of Time. We have said that faunas and 

 floras differ in proportion to the impassableness of the bar- 

 riers between i. e., the height and breadth of the moun- 

 tain-chains, the extent of deserts, and the width and depth 

 of seas, etc. But there is still another element of the 

 'greatest importance, viz., the length of time elapsed since 

 the barrier was set up. This element of time connects 

 geographical faunas and floras with geological changes, 

 and thus geographical distribution of species becomes the 

 key to the most recent of these changes. If we suppose 

 species to undergo very slow changes, then the longer 

 faunas are separated the greater becomes their difference. 

 The full discussion of this important point requires a 

 knowledge of the general laws of evolution, which we are 

 not yet prepared to take up. 



