GEOGRAPHICAL DISTRIBUTION. 159 



only be inferred from the history of the past for which palaeon- 

 tology affords us but slight material. 



This parallel, which naturally presents numerous greater or smaller 

 variations in detail, is explained by the theory of evolution, according 

 to which the developmental history of the individual appears to be 

 a short and simplified repetition, or in a certain sense a recapitulation, 

 of the course of development of the species.' 1 ' 



The historical record preserved in the developmental history of 

 the individual must often be more or less blurred and obscure on 

 account of the many adaptations which have occurred during the 

 embryonic development, or during larval life. Especially in those 

 cases where the peculiar conditions of the struggle for existence 

 demand a simplification, the development will take a more direct course 

 from the ovum to the perfect animal, will be thrown back into an 

 earlier period of life, and finally will be completed before the animal 

 is hatched, until, in absence of a metamorphosis, the historical record 

 is completely suppressed. On the contrary, in the cases of progres- 

 sive transformation where the larval states are gradually modified 

 and live under similar conditions of life, the history of the species 

 will be less imperfectly reproduced in that of the individual. 



Evidence from the Facts of Geographical Distribution. Unlike 

 the facts of morphology, those of geographical distribution raise 

 great difficulties for the theory principally because the phenomena 

 are very complicated and our experiences are still too limited to permit 

 of our establishing general laws. The present distribution of plants 

 and animals over the surface of the earth is clearly the combined 

 result of the earlier distribution of their ancestors and of the geologi- 

 cal changes which have since taken place, the modifications in the 

 extent and position of land and water, which must have had an 

 influence on the fauna and flora. 



Accordingly the geographical distribution of plants and animals f 

 appears intimately connected with that part of geology which has 

 for its aim the investigation of the most recent occurrences in the 

 formation of the earth's crust and its contents. It cannot, 

 therefore, be confined to an examination of the areas of distribution 

 of the animals and plants of the present day, but must take cogni- 

 zance of the distribution of the remains, enclosed in the most recent 

 formations, of the nearest relations and ancestors of living forms, in 



* Fr. Miiller, " Filr Darwin,'' Leipzig, 1864. 



f A.R.Wallace, " The Geographical Distribution of Animals," London, 1876. 

 P. L. Sclater. " Address to the Biological Section of the Brit. Association." 1875. 



