ORGANIC EVOLUTION — DAVENPORT 419 



domestication enables us to see and perhaps preserve such muta- 

 tions. Rather, I should say, the product of such mutation, for the 

 mutation has occurred in the germ plasm before it has become visible 

 in the soma of the organism that develops under the control of the 

 mutated germ pla^m. 



Let us now consider some of the facts of mutation that experi- 

 mental study has revealed. 



Fii-st, mutation is i)r()bably universally occurring in all germ 

 plasms. Thus, in various mammals that have been reared so that 

 they can be observed, mutation has occurred in all visible parts, in 

 internal organs, and in resistance to disease. In man, which is the 

 nuunmal that has been most thoroughly' studied, we have nmtations 

 in hairiness, pigmentation, skin growths, appendages and digits, 

 teeth, sense organs, form of internal organs, like the iliocecal valve, 

 size and functioning of the endocrines, structure and functioning of 

 the nervous system, of the blood and of the reproductive system. 

 Finally, we have mutations in disease resistance, due to obscurer 

 morphological or biochemical idiosyncracies. 



Among pigeons, mutations in color, form of beak, nervous be- 

 havior have arisen in the Whitman-Riddle series. In poultry, I 

 have in the course of 10 years got apparently new nmtations in toes, 

 wings, and nervous reactions. And any poultry fancier knows of 

 the mutations that have occurred in the past 7;"> years in color and 

 pattern, in comb, in cerebral hernia and crest, in feet, wings and beak, 

 and in egg-laying capacity. 



In the insects which have been bred for rapidity of generations 

 mutation has been repeatedly found. In Drosophila^ Mullcr computes 

 that among 500 factors in the X-chromosorae of Drosophila each, 

 in the average, mutates at the rate of one mutation in four years. 

 This would seem to mean that, if you followed a single chromosome 

 and when it divided considered one of the daughter chromosomes and 

 so proceeded through the generations, then at the end of four years 

 the expectation is that in this line of chromosomes some one gene will 

 have mutated and at the end of four more years that gene, or some 

 other in the chromosome line we are following, will have mutated 

 again. But there is an infinitude of chromosomes in the totality of 

 all Drosophila melanog asters. The number in a single gonad is vast ; 

 the number of gonads in the world of Drosophilas that swarm in the 

 autumn over every mass of decaying fruit in a million of orchards as 

 elsewhere is practically infinite. One sees that just Drosophila me- 

 lanogastcr is producing an infinitude of mutations each season, and it 

 has been producing this infinitude aimually for a long time; but time 

 does not count for much, for infinity times a finite number remains 

 infinity. Drosophila throws upon the world each year, a vast num- 

 ber of kinds of mutations in inconceivably great numbers. 



