NATURAL SELECTION: II 499 



more frequently to some mutations than it will to other mutations possible 

 to it. This fact in itself reduces the randomness of the raw materials availa- 

 ble for the construction of evolutionary change. 



Thus it turns out that, strictly speaking, mutations are not random after 

 all. On previous pages we have called them random, but the randomness 

 consists of their occurring without reference to the adaptive requirements 

 of the organisms in which they occur. Conceivably, indeed, a sequence of 

 "most probable" mutations might in itself become a directive force in evo- 

 lution, resulting, perhaps, in a progressive series of changes of the type 

 sometimes cited as an example of orthogenesis (pp. 411-412). The point 

 has not been established, however, and remains at present no more than an 

 interesting speculation. 



In summary, our discussion has shown that past events, by determining 

 at each step in the evolution of life that an organism shall have one struc- 

 ture or attribute and not another, channel and restrict the courses which 

 future evolution may take. At any given time in evolutionary history the 

 raw materials available for the building of further change are not entirely 

 random in nature. Their nature has been determined by preceding events 

 in that history. 



Natural Selection 



Added to this channeling action of past events is the further directive 

 force of natural selection. Previously we have discussed the action of natu- 

 ral selection in sorting the raw materials presented to it; we mention it 

 here to stress the importance of that sorting action as a directive force in 

 evolution. Natural selection always promotes adaptation. Though its ac- 

 tion varies in rate and intensity, its trend is always to cause animals to be- 

 come more perfectly adapted to their environments. Thus it constitutes 

 the main active force directing the course of evolution. 



We might inquire at this point, "Is natural selection a force directing 

 evolving organisms into pathways of progress?" That depends upon how 

 we dehne "progress." If progress consists in becoming more perfectly 

 adapted to the needs of life, whatever they may be, then natural selection 

 always promotes progress. Sometimes progress in this sense may lead to 

 loss of structures possessed by ancestors. Thus parasites frequently lose the 

 sense organs possessed by their free-living ancestors. Is this progress? It is 

 perfection of adaptation, since the sense organs are not needed and their 

 continued formation would constitute waste of the organism's metabolic 

 energies. Hence it is progress of a kind. 



