450 POPULAR SCIENCE MONTHLY 



What, now, is the doctrine of natural selection, as Darwin pro- 

 pounded it? All animals vary; every individual differs from others 

 of its kind, even from its closest kin and from its parents in some or 

 many particulars and to different degrees. Whatever the causes, the 

 fact of variation stands unquestioned. Some variations are of course 

 due to direct environmental influence, and to these Buffon attributed 

 an excessive importance; other deviations from the parental or average 

 specific type are no doubt due to indirect effects of the environment, as 

 Lamarck contended. But there are countless other variations that 

 can not be so explained, some of them indeed appearing before an indi- 

 vidual is subjected to the action of the environment, and these are the 

 congenital variations due to some constitutional even if unknown 

 causes. These seemed to Darwin to be the most important in evolution. 



The second element of the doctrine is that over-production, or 

 rather over-reproduction, is a universal characteristic of living things. 

 The normal rate of multiplication is such that any given form of 

 animal or plant would cumber the earth or fill the sea in a relatively 

 brief period of time. We now know that a bacillus less than %ooo of 

 an inch in length multiplies under normal conditions at a rate that 

 would cause the offspring of a single individual to fill the ocean to 

 the depth of a mile in five days. " Slow-breeding man," wrote Darwin, 

 " has doubled in the past twenty-five years." But excessive multipli- 

 cation is checked by the third part of the whole process, namely, the 

 struggle for existence, that fierce unequal warfare waged by every 

 individual with its inorganic surroundings, with other species of living 

 things, and with others of its own kind. Indeed where members of 

 the same species compete, the struggle often surpasses in ferocity the 

 warfare with other organisms. Communal organisms only are in part 

 exceptions, for in these the battle involves the clash of community with 

 community more than it does the interests of the individuals of a 

 single colony. To what, now, do these elemental processes lead, asks 

 Darwin. Though all seek to maintain themselves, all can not possibly 

 live when only a few can find sustenance or can escape their enemies. 

 Naturally those which possess any advantage whatsoever, that vary ever 

 so slightly in the direction of better adjustment would survive where 

 their brethren perish. And this is nature's selective process, with its 

 positive and negative aspects — the survival of the fittest and the elimi- 

 nation of the unfit. Now we can see why adaptation is a universal 

 characteristic of species — there are no unadapted. If such there were, 

 they have fallen long ago, and the world knows them no more. True 

 it is that perfection is not attained by any creature, but it must estab- 

 lish a modus vivendi or it perishes. Thus, Darwin held, nature per- 

 fects species by dealing directly with favoring derivations that are 

 mainly congenital, and so through these it selects the herditary factors 

 that determine favorable variations. 



