282 



through generation after generation, 

 but is actually increased as a result of 

 the recombinations of the gene com- 

 plexes in their innumerable possible 

 permutations. 



Mutations arc chemical changes in 

 the gene molecule, and since chemical 

 stabilit\- is not absolute, the puzzle 

 about mutations is not so much that 

 thev occur as that they occur so infre- 

 quenth'. Hiis ignorance of the causes 

 which determine the directions in 

 which mutations take place, if such 

 causes indeed exist, is, strange to relate, 

 no handicap to the understanding of 

 the mechanism of evolution, because 

 it is emphatically selection, not muta- 

 tion, that determines the direction of 

 evolution. It has been estimated that 

 if mutation were to stop now, there is 

 already sufficient variation in the plant 

 and animal kingdoms for evolution to 

 continue for as long in the future as it 

 has continued hitherto in the past. 



natural selection, 

 "improbability," and "chance" 



An argument sometimes used 

 against the efficacy of natural selection 

 involves the claim that the initial stages 

 in the evolution of complex structures 

 or functions could not have been fa- 

 vored by natural selection until such 

 structures or functions had reached a 

 certain level of perfection. Like all 

 other arguments of the non possumus 

 type, this one melts away before the 

 progress of knowledge. A case in point 

 is that of the electric organs of fish, 

 developed out of muscles which are 

 capable of discharges strong enough to 

 catch prey and defend the fish against 

 its enemies. These organs are clearly 

 adaptive and confer survival value on 

 their possessors, but the question arises 

 what functions they could perform in 

 the initial stages of their evolution, 

 when it must be supposed that their 

 power was too weak to kill prey or to 



EVOLUTION 



deter predators. Tlie discovery by II. 

 \V. Lissmann that weak electric dis- 

 charges gi\en off by certain fish func- 

 tion in a manner analogous to those of 

 radar equipment, and serve to convey 

 information of the proximity of ob- 

 jects in the water. Electric organs can 

 therefore be adaptive even when they 

 are too weak to kill prey or deter pred- 

 ators. 



It has also been objected that nat- 

 ural selection is a difficult concept to 

 apply to the evolution of ver\' complex 

 adaptations involving co-ordinated 

 variations either in one and the same 

 organism, or even in two different or- 

 ganisms. It is not necessary to go far 

 afield to find examples of this, for in all 

 animals with separate sexes and inter- 

 nal fertilization there has been a sepa- 

 rate yet harmonious evolution of the 

 reproductive organs in the two sexes. 

 It has been supposed that such situa- 

 tions argued so high a degree of 

 "mathematical improbability" that 

 they could not be explained as a re- 

 sult of natural selection, which was, 

 ver\' erroneously, called "chance." 



Those who invoke mathematical 

 improbability against natural selection 

 can be refuted out of their own mouths. 

 Muller has estimated that on the exist- 

 ing knowledge of the percentage of 

 mutations that are beneficial, and a 

 reasoned estimate of the number of 

 mutations that would be necessary- to 

 convert an amoeba into a horse, based 

 on the average magnitude of the ef- 

 fects of mutations, the number of mu- 

 tations required on the basis of chance 

 alone, if there were no natural selec- 

 tion, would be of the order of one 

 thousand raised to the power of one 

 million. This impossible and meaning- 

 less figure serves to illustrate the 

 power of natural selection in collecting 

 favorable mutations and minimizing 

 waste of variation, for horses do exist 

 and they have evolved. 



From the undoubted fact that 



