VARIATION, SELECTION AND MUTA- 

 TION IN ONE OF THE PROTOZOA 



THERE is a theory that the one- 

 celled organisms dividing as they 

 do by simple fission ought to be 

 all alike in their heredity sub- 

 stances, in other words that the geno- 

 type should be constant, and selection 

 should be of no avail. The recent 

 work of Middleton and Jennings has 

 brought this theory into question and 

 still more recently (1918) R. W. Hegner 

 has made a contribution to this funda- 

 mental biological problem. In his in- 

 vestigation Professor Hegner used one 

 of the lowest and simplest of animals 

 but one that fortunately has a variable 

 characteristic that readily lends itself 

 to accurate observation, namely the 

 number of spines on its shell. His 

 conclusions and proofs are clearly given 

 in Volume 4 of the Proceedings of the 

 National Academy of Sciences. In 

 summarized form they are as follows: 

 Arcella dentata is a microscopic 

 protozoon belonging to the lowest 

 class, the Rhizopoda. It is as simple 

 as any animal organism it is possible 

 to obtain that has measurable charac- 

 teristics. It varies in diameter from 73 

 microns to 150 microns and in spine 

 number from 7 to 20. It multiplies 

 vegetatively and rapidly and the char- 

 acteristics of the shell are not modified 

 by growth or by the environment, and 

 are heritable but variable. Can heri- 

 tably diverse lines with respect to spine 

 number and diameter of shell be dis- 

 tinguished among the descendants of a 

 simgle specimen of Arcella dentata pro- 

 duced by simple fission? 



Small families were reared from 

 70 "wild" specimens selected so as to 

 include large, small, and medium sized 

 organisms. Seven hundred and forty- 

 six specimens were obtained in this 

 way, ranging in number from only 2 or 

 3 to 149 per family. The mean spine 

 number of the families ranged from 

 10.40 to 14.07. Variations in spine 



number occurred among the descendants 

 of single specimens during fission and 

 these variations were in part inherited. 

 It was found that the hereditary con- 

 stitution of the different families was 

 different with respect to spine number 

 and the conclusion was reached that a 

 "wild" population consists of a large 

 number of heritably diverse families so 

 far as spine number is concerned, and 

 also probably as regards diameter, since 

 spine number and diameter are closely 

 correlated. 



The main problem was next under- 

 taken, i. e., an attempt was made to 

 isolate heritably diverse lines from 

 among the descendants of a single speci- 

 men produced by vegetative reproduc- 

 tion. 



The data prove that the descendants 

 of a single specimen of Arcella dentata 

 produced by vegetative reproduction 

 differ slightly from one another in 

 their hereditary constitution and that 

 heritably diverse lines may be iso- 

 lated from among them, differing 

 both in spine number and in diameter, 

 and that these two characters are closely 

 correlated. These heritably diverse 

 lines resemble certain of the families that 

 were reared from "wild" specimens, 

 and suggest that differences in the 

 hereditary constitution of these wild 

 specimens may have originated in the 

 same way. 



The general conclusion reached is 

 that within a large family of Arcella 

 dentata produced by vegetative repro- 

 duction from a single specimen, there 

 are many heritably diverse branches. 

 These diversities are due both to 

 very slight variations and to sudden 

 large variations ("mutations"). The 

 formation of such hereditarily diverse 

 branches appears to be a true case of 

 evolution that has been observed in the 

 laboratory and that occurs in a similar 

 way in nature. 



143 



