CHAPTER XXXI 



THE FAILURES OF ARBOREAL LIFE 



THERE would seem to be a general law applicable to 

 animal adaptations a law which we might term the 

 law of successful minimal adaptive specialization. A 

 plastic stock, given unlimited scope of development in 

 varied environment, tends to differentiate. Different 

 races will specialize towards the needs of their environ- 

 ment. Different environments offer varying possibilities 

 of education, expansion, and advance, but the full educa- 

 tional possibilities are not necessarily grasped solely, or 

 to the full, by the animal which becomes most completely 

 specialized. This is a fact made clear by a whole sequence 

 of geological types which have seized upon their environ- 

 mental opportunities, and have become specialized in 

 an extraordinary degree to fit their environment, only 

 to arrive at specific senility, and be supplanted by less 

 specialized and more plastic types. A complete, early, 

 and all-absorbing specialization is almost synonymous 

 with specific senility. An animal which specializes to 

 the limits, in response to its environment, becomes a 

 slave to its environment, and loses its greatest evolution- 

 ary asset of plasticity. This, in the end, spells the doom 

 of progress. It does not matter greatly in what venture 

 the all-absorbing specialization is cultivated. It may be 

 in response to environment; it may be in protective 

 mechanisms, it may be in diet. As Willey has said, 

 " Hardly anything proclaims a finished organization, the 

 culmination of a phyletic career, so plainly as an exclusive 

 diet." 



A specialization for blood-sucking, a specialization for 

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