542 - Heredify and Evolution 



been made in the direction of the faster 

 growing seedlings, and the selecting agency 

 has been the environment. Similarly among 

 animals, the primary selecting factor is very 

 frequently a competition for the limited sup- 

 ply of organic food; and since individuals of 

 the same species have the same food require- 

 ments, the struggle is keenest within such a 

 population. 



Of course there are wholesale and indis- 

 criminate forces of destruction (storms, floods, 

 glaciers, etc.), which may reduce a popula- 

 tion without any regard for the capacity of 

 the individual to survive and reproduce. 

 Also an individual's survival may hinge upon 

 sheer accident. Some seeds fall on good 

 ground and some on stony ground — through 

 no virtue of the former or fault of the latter. 

 But over and above such nonselective elimi- 

 nations, speaking i» terms of the average, the 

 individuals of a species that are most healthy, 

 vigorous, capable of securing food and escap- 

 ing or protecting themselves from enemies, 

 resistant to disease and unfavorable weather, 

 and so forth, and so forth — in short, those 

 that are fittest to survive and reproduce 

 under the given conditions — are most likely 

 to be the perpetuators of the species. Thus 

 variations — or more particularly variations 

 of the heritable type — play a very definite 

 role in survival by providing the material 

 upon which the environment exerts its se- 

 lective action. 



Natural selection differs from artificial 

 selection not only as regards the criteria by 

 which the surviving individuals are chosen, 

 but also as to the time element. Progress 

 ceases in an artificially selected line when 

 all genes favorable to the chosen character- 

 istics have been collected in a group of pure- 

 bred individuals. This is because man does 

 not have time to wait for "lucky" mutations 

 in a desired direction. But natural selection 

 has no such limitation. Natural selection has 

 "all the time there is." Generation after 

 generation, century after century, millen- 

 nium after millennium, the struggle for 

 existence goes on, selecting the fittest and 



eliminating the less fit. A great majority of 

 mutations, probably, are soon eliminated be- 

 cause they are unfavorable, and many muta- 

 tions may be indifferent as to their survival 

 value. But some favorable mutations — espe- 

 cially of the slight type — are bound to appear 

 periodically. Even favorable mutations may 

 accidentally be eliminated; but on the aver- 

 age, if a mutation gives any advantage to its 

 possessors, the mutant individuals will have 

 a greater chance than the nonmutant indi- 

 viduals of surviving and transmitting the 

 new trait to their descendants, and in the 

 next generation, the mutants, in turn, will 

 be similarly favored in the struggle for 

 existence. In the long run, therefore, every 

 mutation that is favorable to survival or 

 reproduction stands a good chance of being 

 permanently incorporated in the genetic con- 

 stitution of the species. 



Artificial and natural selection differ, 

 broadly, in that artificial selection operates 

 chiefly by the more rapid but limited meth- 

 od of selecting favorable recombinations of 

 existing genes; while natural selection oper- 

 ates by the slower but potentially unlimited 

 method of selecting favorable new muta- 

 tions. To some extent both methods operate 

 in both cases, since both the artificial and 

 the natural processes actually select the sur- 

 viving parents on the basis of their pheno- 

 typic characteristics, regardless of how these 

 arose. The great difference lies in the time 

 factor; artificial selection has operated for 

 only a few centuries, at most, but natural 

 selection has gone on for millions of years. 

 Natural selection proceeds in a bungling, 

 haphazard, hit-or-miss fashion, and therefore 

 it is very slow. But given an infinity of time, 

 even such a slow and halting process can 

 produce tremendous results — as is witnessed 

 by the great diversity of living forms inhab- 

 iting our earth today. 



Even on a short-term basis mutational 

 changes may be of great selective value to a 

 species, as is shown by the recent work of 

 Milislav Demerec at the Carnegie Institution 

 in Cold Spring Harbor, N.Y. Demerec found 



