450 AN INTRODUCTION TO ZOOLOGY 



why they should not appear in all parts of the world. Whereas we 

 find in Madagascar, Australia and New Zealand, which have been 

 separated from the nearest land for long periods of time, that they 

 have a number of animals distinctive of themselves and not found 

 elsewhere. In the case of Australasia we find, for example, that the 

 lowest mammals, Echidna and Ornithorhynchus , are limited to that 

 region and found nowhere else. When we do find species of animals 

 that are widely spread there is an explanation forthcoming. For 

 example, marine forms like some fish live in the sea, which does not 

 present so many barriers as the land ; or birds, butterfles, etc., being 

 winged forms, can overcome obstacles that would prove barriers 

 to other less easily moving forms. 



Apparent contradictions to this rule are to be met with in certain 

 groups, e.g. the wingless birds which appear in New Zealand (the 

 Kiwi), Australasia (the Emu and the Cassowary), Africa (Ostrich), 

 and South America (the Rhea or South American Ostrich) ; and 

 again, the Marsupials or pouched mammals which occur in Austral- 

 asia and South America. The explanation of this seeming anomaly' 

 is that both groups originated in or near the land masses of the 

 Northern Hemisphere and spread widely, as their fossil remains show. 

 Later on more highly developed forms appeared on the old land area, 

 and the competition drove the earlier forms to more and more remote 

 places. The Northern expansion was limited by climatic factors, and 

 the result was that they were driven into the southern projecting 

 points, where they have remained. In the case of the marsupials 

 in Australia, the land connections broke down before the higher 

 mammals reached it, so that the marsupials became the dominant 

 forms. Hence we see that the apparent exceptions in reality only 

 add a further proof of this gradual change. 



Palceontological Evidence. 



If evolution has actually taken place, and we could obtain 

 specimens or records of past animals and plants, then of course 

 we should be able to clinch the argument by showing the actual 

 series of modifications. Such records are fortunately preserved 

 to a certain extent in the actual remains or other evidences of 

 pre-existing animals and plants embedded in the rocks of the earth, 

 and to which we give the name of fossils. Had examples of all past 

 animals been preserved, it would then be a straightforward though 

 endless task to show how all forms were related to one another and 

 bridge the gaps at present separating them, by intermediate forms. 



A little thought will show that we cannot expect the geological 

 record to be complete. Only a small part of the surface of the earth 

 is possible of access, and only an infinitesimal part of this has been 



