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Alier (Jruenljera. 'J'lie iiluii/ ut Evulution 



AN INFANT'S RESEMBLANCE TO ITS ANCESTORS 



The almost shapeless Sacculina (C) growing as a parasite on the abdomen of the crab 

 (D) has nothing in its structure or behavior to suggest a relationship to its host. Yet 

 in its early development (A, B) it seems destined to become an unmistakable crus- 

 tacean. Without a study of its life history we should never have guessed that host 

 and parasite are of the same class of animals 



Darwin pointed out that similar regions which are occupied by different 

 flora and fauna are always separated from each other by impassable barriers, 

 such as oceans, mountain ranges, and deserts. On the other hand, when 

 regions which differ markedly in climate, soil, and so on are inhabited by 

 similar plants and animals, they are either directly connected at present or 

 show evidence of having been so connected in the past. For example, the 

 plants and animals found on oceanic islands are frequently quite distinct from 



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