18 The Animal Ecology of the Cold Spring Sand Spit 



traca. The prawns of the sea are replaced by a very closely related species, Palae- 

 monetes exilipes, which may have gained the great lakes by the way of the Missis- 

 sippi river, in which it is abundant. The marine lobster is replaced by the crayfishes 

 (Cambarus propinquus and C. virilis). The predaceous forms are the fishes which 

 feed largely upon the snails and the Crustacea. 



II. FAUNA OF THE BEACH 



In the tideless lake the lower and upper beaches are hardly to be distinguished. 

 On the lake strand Collembola are found just as on the lower sea beach. In the line of 

 debris that the waves deposit are found the wrecks of all the shallow -water forms of 

 which I have spoken, and, in addition, the carcasses of vast numbers of insects that 

 have fallen into the lake, have drowned, and are cast up by the waves. This wreck- 

 age line affords, then, just the feeding-ground for inland species that the marine 

 species find on the coast. What animals do we find here? The burrowing Orches- 

 tidse seem, indeed, to be absent, but there is a closely related species (Allorchestes) 

 that lives in the shallow water. That it is not a beach burrower may be due to just 

 these causes that have eliminated the burrowing habit in general. But under the 

 debris rove-beetles and insect larvae are to be found feeding on the vegetable matter/' 

 And small red ants build nests on the beach and visit the debris for the carcasses of 

 insects. A similar carrion fly and carrion beetle (Necrophorous) occur. Feeding 

 upon this fauna is a running spider (Lycosa cinerea) the same as that of the coast. 

 Here, too, occur robber flies and tiger beetles, and even white grasshoppers. Thus 

 the lake beach, having a similar strand zone of decaying vegetation and plant wreck- 

 age with the sea, has, at a distance of nine hundred miles from the sea, excepting 

 certain strictly marine species, practically the same fauna as the sea. The conclusion 

 to be drawn from this fact is the immense importance of habitat (t. e., of environ- 

 mental details) in determining similarity of fauna, or, in other words, the fauna of a 

 point is, within limits, determined rather by the environmental conditions than by the 

 geographical position of the point. 



REMARKS ON THE THEORY OF ADAPTATION 



Everyone must admit the fact of adaptation of the structures of animals to their 

 environment. The generally accepted theory to account for this is that of Darwin 

 and Wallace that a species coming into a new habitat gradually acquires a fitness to 

 that habitat by the killing off of the less fit individuals born into the species. There 

 are some cases, as, for instance, that of the leaf insects, that of the fungus beetle, 

 and those of mimicry, that I can see no other explanation for but this, that an exter- 

 nal condition existed first and a structure or coloration was acquired by the race that 

 fitted or adjusted it to that external condition. 



We must not, in accepting any theory as a true one, try to force it as a universal 

 theory and become blinded to other possible theories. Now there is another and f unda- 



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