CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 347 



than any other fauna and flora. 3. Climate, soil, and position 

 explain all the phenomena of distribution. 



II. Theory of Evolution, generally accepted. Eegards modern 

 distribution as the result of innumerable changes that have 

 affected (1) organisms; (2) the surface of the globe. 



1. Geological history shows that there has been a succession of 

 faunas and floras passing gradually into one another, old species 

 becoming extinct, and new ones being evolved by the combined 

 influence of variation and heredity, which respectively originate 

 and accumulate new characters. Upon the whole a gradual 

 advance in complexity has taken place, but the geological record 

 is extremely imperfect, especially as regards land organisms. The 

 process of change is still going on, and examples of modern 

 extinction (e.g., the gigantic wingless New Zealand bird Dinornis 

 and Sirenian form Rhytina) are well known, but the detection 

 of newly evolved species involves greater difficulties. 



Species once established have extended themselves over smaller 

 or larger areas, according to their powers of migration and sur- 

 rounding conditions i.e., their environment. Physical barriers, 

 such as oceans, mountains, climate, and soil, have played an 

 important part in limiting such extension, but the competition 

 of other forms has had a still greater influence. Introduced 

 forms often increase prodigiously, and even supersede the indi- 

 genous ones, whence it follows that these last are not necessarily 

 the best adapted. Exs. Rabbits in Australia; the brown 

 rat in England, which has almost ousted the indigenous black 

 rat. 



2. Owing to the wearing away or erosion of the land by 

 various agencies (chiefly the different forms of water), and the 

 action of subterranean forces by which upward and downward 

 movements of the earth's crust are produced, the distribution of 

 land and sea has constantly varied. Europe and North America, 

 for example, have most likely been connected at various times 

 by land occupying part of what is now the North Atlantic, and 

 Australia appears to have been once united with Asia. On the 

 other hand, evidence is found on every continental land surface 

 of the former presence of the sea. In spite of what has been 

 said, the theory of " permanence of oceanic and continental 

 areas " finds much support. According to it the great oceans are 

 of extreme antiquity, and, on the whole, more or less land has 



