( 512 ) 



and dissimilarity in the characters of representative forms, it wUl be evident that the 

 difference iu the fauna of neighbouring districts is dne to two kinds of eutirel}' 

 dift'erent factors, geological and biological factors, and that the study of the 

 geographical distribution of animals and plants consists of two branches — the 

 geological branch, which has to do with sucli discrepancies in the fauna and flora 

 as allow conclusions to be drawn as to the geological transformation of the 

 districts in question, and the biological branch, which has to do with those other 

 differences from which we can draw conclusions as to the transformation of the 

 animals and plants in question. And now we can extend our former conclusion that 

 differences in the characters of representative forms which stand in close phylo- 

 genetic connection are an expression of the differences in the biological factors of 

 the districts concerned, to those forms which are representatives in a biological 

 sense and exclude each other in conse()n('nce of a similarity in habits, and again to 

 those which cannot exist beside one another in the same district, because they are 

 so different in habits that they require a resj)ectivply different environment. It is 

 certainly conceivable that the absence of woodpeckers, which abound in the Indo- 

 Malayan Region, from the lesser Snnda Islands (and all the islands farther east) 

 could be due to the presence of cockatoos, which are, like those, hole-breeders, though 

 we do not maintain that this similarity in habits is tlie actual reason of the two 

 widely different groups of birds excluding each other in that region ; while, again, 

 the discrepancy in the composition of the West African from the East African fauna 

 is readily explained by the former being a wet forest country, the latter an open and 

 dry country, and the affinities which the West African forest region has in the fauna 

 with the ludo-Malayan Region, and the analogous affinities of East Africa with 

 Western India, are consequences of similar physiographical conditions of the 

 countries, just as the difference in the characters of the West African, East 

 African, and Malagassic morphologically allied representative forms — which I shall 

 call morphological representatives, as distinctive of biological representatives — are 

 due to differences in the transmuting factors of the respective districts. The con- 

 trast in which these explanations of faunistic discrepancies stand to the explanation 

 of those discrepancies which are caused by geological factors is strikingly illus- 

 trated by a comparison of the fauna of Madagascar witli that of Africa, iu so far as 

 we have rightly to conclude from the absence of the large Carnicora ami Ungidata, 

 so abundant iu Africa, from Madagascar tluit this island must already have been 

 isolated from Africa by a wide sea-arm at tliat time when those animals immigrated 

 from the North into the Aethiopian Region. 



The difference in tlie fauna of various districts is further dependent on the fact 

 that different animals are, as said before, affected by the transmuting factors of a 

 certain locality in a different degree, so that often some forms are, while some are 

 not, modified in characters. We have consequently to take into account also the 

 physiological constitution of the animals when treating upon zoogcographical 

 questions. Hence it is evident that the result of the division of the earth's surface 

 in zoogcographical areas must be ditt'erent according as we take the faunistic dis- 

 crepancies depending on the geological factors as the base of division, or those 

 differences which are caused by the biological factors; and again, if we take the 

 biological division, it is obvious that tlie extent of the areas must be quite different 

 according as to which animals are taken into consideration. 



Although Wallace has emphasised, throughout his Geographical Distribution of 

 Animals, the high importance of tlie geological branch of zoogeography relating to 



