I] HISTORICAL 9 



enormous changes in the configuration of the con- 

 tinents and oceans because the theory of descent, with 

 its necessary postulate of great migrations, required 

 them. 



In the same year Moritz Wagner showed that 

 migration implies not only new environmental con- 

 ditions but also secures separation from the original 

 stock and thus eliminates or lessens the reactionary 

 dangers of panmixia or promiscuous interbreeding. 

 This idea had been discovered before and it has been 

 rediscovered several times after Wagner. Through 

 the heated discussions of the more ardent selection- 

 ists, Wagner's theory came to grow into an alternative 

 instead of a great help to the theory of selectional 

 evolution. Separation is now rightly considered a 

 most important factor in the making of new species. 

 C. Semper could well say in his suggestive work 

 Existenz-Bedingungeti der Thiere, also published in 

 the fertile year 1868, that ' our whole Zoogeography 

 is indeed nothing but a big heap of bricks, piled up 

 without sense,' and he was one of the first to eluci- 

 date the distribution of a group Holothuria or Sea 

 Cucumbers by tracing the relationship of its genera 

 and species, whilst distinguishing between their 

 centres of origin and their subsequent migrations, 

 and not losing sight of former geological conditions. 



In 1872 Alexander Agassiz of Harvard followed 

 with a morphological systematic revision of the Sea 



