PRINCIPLES OF DISTRIBUTION. 227 



PRINCIPLES OF DISTRIBUTION. 



Explanatory Theories 



I. Theory of Special Creations, now abandoned. According to it: 1. Organisms 

 were created where now found. 2. The fauna and flora of any particular region must be 

 better adapted to it than any other fauna and flora. 3. Climate, soil, and position 

 explain all the phenomena of distribution. 



II. Theory of Evolution, generally accepted. Regards 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 organ- 

 isms. The process of change is still going on, and examples of modern extinction (e.g. 

 the New Zealand Dinornis and Sirenian form Rhytina) are well known, but the detec- 

 tion 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 surrounding conditions, i.e., their environ- 

 ment. 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 indigenous 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 laud 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 

 lohole, more or less land has existed from very remote times within the present continental 

 areas. It is perhaps best to accept this theory only for the deeper parts of the great 

 oceans. An accurate knowledge of the contours of the ocean-floor is important in this 

 r.onnection, and serves as an important check upon speculations regarding former land- 

 unions. On this basis islands have been divided into oceanic and continental, which are 

 believed respectively to have been always isolated, and to have been connected with an 

 adjoining continent. 



Oceanic Islands are : (a) generally remote from continents ; (6) separated from them 

 by very deep (usually over 1000 fathoms) water ; (c) of volcanic or coral nature ; (rf) 

 inhabited by forms which possess powers of migration capable of carrying them, actively or 

 passively, over more or less broad ocean tracts ; (e) characterised by numerous peculiar 

 species. Exs. The Azores, St. Helena, Ascension, coral islands of Pacific. 



Continental islands are: (a) comparatively near a continent ; (b) separated from it by 

 comparatively shallow (under 1000 fathoms; water ; (c) of similar geological structure, 

 and not entirely volcanic or coral ; (d) inhabited by similar organisms, irrespective of 

 powers of migration. Such islands are : (1) Ancient continental, separated from the 

 nearest continent by fairly deep (over 100 fathoms) water, and presenting only a general 

 resemblance in the fauna and flora ; many peculiar species. Exs. Madagascar, Malay 

 Islands. (2) Recent continental, separated from the adjacent continent by shallow (not 

 more than 100 fathoms) water, and with closely similar fauna and flora ; very few peculiar 

 species. Exs. British Islands, Japan. Both (1) and (2) have presumably been united 

 with the adjoining continents, the latter at a recent date, geologically speaking. 



The surface of the globe has also undergone numerous mutations as regards climate. 

 The temperate parts of N. America and Europe, for example, were at a geologically 

 recent period passing through a glacial epoch (the " great ice age"), as proved by ice- 

 worn and scratched rock-surfaces and rocks, boulder-clay. &c. On the other hand, fossil 

 plants evidencing subtropical conditions have been found in the Arctic regions. Many 



