Page eight 



EVOLUTION 



June, 1931 



the multiplication rate of the ancestors of man would have led 

 to, if all the descendants of these ancestors had continued 

 multiplying at this same rate, i.e., without selection. 



It might here be inquired whether such a rate of multiplica- 

 tion would have been possible or likely to occur in these an- 

 cestors, in the time during which life has existed on the earth. 

 We know that life has been here for a period having an order 

 of magnitude of something like a thousand million years, that 

 b, a million millennia. If there were only 1,500 mutational 

 steps in this time, that would make only one step in each 670,- 

 000 years. Our postulate, r — 100, requires that an individual 

 in the line of descent of man should multiply at least a hun- 

 dredfold between each advantageous mutational step that be- 

 came incorporated in the germ-plasm, and the next one. It 

 is obvious that far more than this much multiplication could 

 easily happen in 670,000 years. For it only takes seven doub- 

 lings to make a hundredfold multiplication, and the slow-breed- 

 ing modern European has been able to double his population 

 merely in the space of the last century. The multiplying or- 

 ganisms, then, would have no difficulty in fulfilling these con- 

 ditions. 



Suppose, now, we try the more extreme figures, r — 10,000 

 an 5—1,000,000, so that r= becomes (10,000) i'""""'"'. To go 

 through a million mutational steps in the course of a million 

 millennia would require one mutation to become incorporated in 

 each millennium, or thousand years. It would also be necessary 

 for the selected tyjje of mutant to multiply by 10,000 during 

 this period of time, and meanwhile to undergo another muta- 

 tion. There can be no reasonable doubt that a millennium is 

 plenty long enough for many another mutation to occur, in 

 all the descendant germ-plasms, but how about the large amount 

 of multiplication here required? Most lower organisms go 

 through a generation in not over a year's time, and are able, 

 when given the opportunity, to multiply many fold in a single 

 generation. If, however, we suppose that the "select" indi- 

 viduals, those with "good" mutations, only increase in numbers, 

 on an average, by 2 per cent, in each generation, then, at a 

 year to a generation, each such individual would increase from 

 unity to nearly two hundred million in the course of a thousand 

 years. This is far beyond our requirement of 10,000 times. 

 Thus we see that the multiplying organism could probably do 

 much better than accumulate 1,000,000 mutations during the 

 time that life has already existed here, even though each muta- 

 tion represented the selection of the best in 10,000. Allowance 

 must, however, be made for the fact that accidental elimination 

 wipes out the great majority of mutant genes within a few 

 generations after their origination. That is, the process of 

 "differential multiplication" or "selection" is very haphazard 

 until a sizeable number of individuals with the mutant gene 

 happens to become established. 



There is another process which works in the opposite direc- 

 tion to the above, i.e., which hastens the "establishment" of 

 advantageous mutations in the selected lines of descent. This 

 process is the formation of new combinations of genes occur- 

 ring in sexual reproduction. For the sake of simplicity it has 

 been ignored in this account. By its means it is made possible 

 that various different advantageous mutant genes which have 

 been multiplying simultaneously in parallel, in as many dif- 

 ferent (but partially overlapping) sections of a population, 

 can be finally combined into one line of descent. Thus many 

 more mutant genes can be accumulated into one (final) line 



of descent, in a given length of time, than if all the mutational 

 events and selections had to occur successively in a single line. 

 Owing to this factor, the number of mutational steps may well 

 have been of a considerably higher order of magnitude than 

 1,000,000. Multiplication hence has probably afforded the 

 opportunity of obtaining an individual that represents a 

 chance of even less than one in (10,000)'°°°°°'. 



NATURAL SELECTION PRUNES LIFETREE 



It should be noticed that, for the evolution of the multiply- 

 ing organisms, the only two required conditions have been the 

 occurrence of "chance" mutations (which need include only a 

 very minute proportion of "good" ones) , and the ability of the 

 individuals carrying the "good" mutations to multiply to an 

 extent which, within the limits of one generation, need be only 

 extremely limited, but which, continued over a great lapse of 

 time by something akin to a geometric progression, becomes 

 prodigious indeed. In this process the role of "natural selec- 

 tion" consists in just this: that by the elimination of the "unfit" 

 individuals, or the restriction of their numbers, room is made 

 to alloif the multiplication of the others at the rate required to 

 provide the "chance" for the remarkable 1 in (100)^^"" or 1 in 

 (10,000) I'""''""' combmation to appear. In other words, selec- 

 tion merely gives opportunity for the multiplication to proceed 

 in the adaptive or better-organized lines at such a rate as would, 

 if uniformly continued throughout, have given the total which 

 automatically contains the "desired" combination. At the same 

 time, we should not minimize the importance of natural selec- 

 tion in determining which individuals will be allowed to multi- 

 ply, and, therefore, which of the myriads of possible directions 

 evolution will be allowed to take. The old analogy to the pro- 

 cess of pruning a tree is very pertinent in this connection. 

 If we imagine a world in which, through some sort of miracu- 

 lous intervention, the combinations which we now call the "un- 

 fit" are all allowed to persist and reproduce like the others, the 

 evolution of the "fit" would nevertheless proceed much as in 

 our own world, so long as they too were granted the oppor- 

 ranity to multiply as they do here. Thus "natural selection" 

 would not be necessary for their production. But these "fit" 

 or "well-organized" individuals, and lines of individuals, though 

 in absolute numbers as numerous as here, would necessarily 

 form but an infinitesimal fraction of all the unthinkably vast 

 horde of other combinations that had come into existence 

 simultaneously (just as in the hypothetical case of the non- 

 multiplying beings, in which, if we started with this same final 

 number to begin with, we would eventually find included 

 among them by sheer accident creatures as complexly adapted 

 as ourselves). The fact that the fit owed their existence to 

 "chance" would then be obvious, owing to the relative small- 

 ness of the minority in which they existed. In our world, the 

 misfits are largely nipped in the bud, and yet, in the sense 

 just explained, we see that we too are really but the vanish- f^ 

 ingly small, viable, visible fraction of a stupendous ghostly 

 army of potential creatures, involving a total of (100) *^'"' to 

 (10,000) '■"""■"'"', or more, possible combinations of misfits. 



A little consideration may now be given to the size of this 

 theoretical total number, to show that actually it would be quite 

 impossible of physical attainment. Consequently, if there were 

 no selective elimination, multiplication could not possibly have 



