The wanderings of animals / by Hans Gadow.

  • Hans Friedrich Gadow
Date:
1913
Catalogue details

Licence: Public Domain Mark

Credit: The wanderings of animals / by Hans Gadow. Source: Wellcome Collection.

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    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-Bedingungen der Thieve, 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