so THE PROBLEMS OF EVOLUTION 



pigeons, it is diflScult to comprehend, but this is 

 no proof that it is imaginary. 



The old analogy of the stream is sound. We have 

 no doubt of the existence and reality of the Missis- 

 sippi River, yet it is beyond our power to see it as 

 a whole. We may see a small part of it by standing 

 on its banks, but at no successive moments does 

 that small part have exactly the same character- 

 istics. So too the species exists as an ever changing 

 aggregation of individuals, their limits beyond our 

 view, their changes constant and usually elusive, 

 but always within a boundary which our fragmen- 

 tary knowledge permits us to realize, as we map the 

 course of a stream. 



Which is more real, the Mississippi River or a 

 pint of water dipped from it.^ It does not matter 

 that it differs in flood and in drought, nor that it is 

 always in motion; the river exists. Likewise the 

 species, although it is a troublesome thing to fix, 

 exists always as an aggregation of individuals, 

 sometimes of various kinds in form or functions, 

 fluctuating both in space and in time, but always 

 associated either structurally or functionally and 

 always part of a reproductive sequence. 



Since we accept the reality of evolution, we must 

 recognize the inconstancy of species through time. 

 In this respect there can be no definite boundaries 

 of species, save only the abrupt limit established 

 by extinction. Consequently the requirements of 



