296 GEOGKAPHICAL DISTRIBUTION. [CHAP. XIL 



history have beaten their predecessors in the race for life, and 

 are, in so far, higher in the scale, and their structure has generally 

 become more specialised ; and this may account for the common 

 belief held by so many palaeontologists, that organisation on the 

 whole has progressed. Extinct and ancient animals resemble to 

 a certain extent the embryos of the more recent animals belonging 

 to the same classes, and this wonderful fact receives a simple 

 explanation according to our views. The succession of the same 

 types of structure within the same areas during the later geo- 

 logical periods ceases to be mysterious, and is intelligible on the 

 principle of inheritance. 



If then the geological record be as imperfect as many believe, 

 and it may at least be asserted that the record cannot be proved 

 to be much more perfect, the main objections to the theory of 

 natural selection are greatly diminished or disappear. On the 

 other hand, all the chief laws of palaeontology plainly proclaim, 

 as it seems to me, that species have been produced by ordinary 

 generation: old forms having been supplanted by new and 

 improved forms of life, the products of Variation and the Survival 

 of the Fittest. 



CHAPTER XII. 

 GEOGRAPHICAL DISTRIBUTION. 



Present distribution cannot be accounted for by differences in physical con- 

 ditions Importance of barriers Affinity of the productions o'f the same 

 continent Centres of creation Means of dispersal by changes of climate 

 and of the level of the land, and by occasional means Dispersal during 

 the Glacial period Alternate Glacial periods in the North and South. 



IN considering the distribution of organic beings over the face of 

 the globe, the first great fact which strikes us is, that neither the 

 similarity nor the dissimilarity of the inhabitants of various 

 regions can be wholly accounted for by climatal and other 

 physical conditions. Of late, almost every author who has 

 studied the subject has come to this conclusion. The case of 

 America alone would almost suffice to prove its truth ; for if we 

 exclude the arctic and northern temperate parts, all authors agree 

 that one of the most fundamental divisions in geographical distri- 

 bution is that between the New and Old Worlds ; yet if we travel 

 over the vast American continent, from the central parts of the 

 Unit 3d States to its extreme southern point, we meet with the 

 most diversified conditions; humid districts, arid deserts, lofty 



