A. DEFINITION OF EVOLUTION 



result, not only the physical surroundings but also the biotic envii'onment 

 of the mountain colonizers will be different, and so natural selection will 

 favor modification of the colonists. But these should bear within their 

 structure and habits evidences of their close relationship to their lowland 

 progenitors. On the other hand the mountain colonists of different regions 

 will be different because of the long isolation of their ancestors. Similar 

 considerations apply to the colonization of any new territory whatever. 

 And finally, the fossil remains in any region should resemble the living 

 organisms of the same region, more closely in the case of recent fossils, 

 less closely in the case of ancient fossils. This is required by the rather 

 obvious fact that the present inhabitants of any region must have been 

 descended from the past inhabitants. Great migrations of the past may 

 modify the truth of this proposition in some instances, but they cannot 

 take away its general validity. Darwin was quoted above for one example, 

 namely the fossil armored mammals of South America which closely re- 

 semble the present armadillos of the same continent. Further discussion 

 of this important topic may be deferred to the chapter dealing with pale- 

 ontological evidences of evolution. All of the above facts follow logicallv 

 from the Darwinian hypothesis, yet each is anomalous if it be assumed 

 that each species has been independently created in its present range. 



DISTRIBUTION OF FRESH-WATER ORGANISMS 



Darwin regarded fresh-water organisms as the most noteworthy exception 

 to the principle that organisms separated by a barrier are quite different. 

 River systems and lakes are, of course, separated from one another by 

 barriers of land. While many fresh-water systems frequentlv emptv into 

 the same ocean, salt water is a barrier to most fresh-water org-anisms 

 which is no less formidable than land. Hence one might expect an unusual 

 degree of differentiation in fresh-water floras and faunas. But the opposite 

 is the case. There is great similarity between fresh-water organisms 

 throughout the world, and many individual species are world-wide in 

 distribution. Darwin believed that this could l)e accounted for by the fact 

 that most fresh-water organisms must, in order to survive, be adapted for 

 frequent short migrations from pond to pond or from stream to stream 

 within a limited locality. But these migrations will inevitably lead to 

 longer ones occasionally. Given time on a geological scale, this should re- 

 sult in very widespread species. 



Darwin gave much attention to what might be called accidental means 

 of transport of fresh-water organisms. A very common phenomenon is the 

 joining of different riv^ers and lakes by their floodwat(M-s in the spring. 

 This should permit a very extensive exchange of their inhabitants. More 

 selective and certainly less common is the transport of fishes and other 

 small organisms by whirlwinds and tornados. A whirlwind, when passing 

 over water, may pick up the surface waters together with anv small organ- 

 isms which happen to be near the surface. Later, when the force of the 

 wind abates, the water and its contents will be dropped. If it should hap- 

 pen to drop over another body of water, the organisms so transported 



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