96 



The Journal of Heredity 



species of small animal ( Paramecium) 

 which ordinarily multiplies b>' dividint; 

 into two parts, each of w^hich becomes 

 an entire individual. Only at rare in- 

 tervals does it reproduce by a method 

 comparable with sexual reproduction in 

 the higher animals and plants. If 

 variation in genetic factors arises only 

 from sexual reproduction, then asex- 

 ually produced lines of Paramecium 

 should be devoid of genetic variation 

 and so unaffected by selection. Jen- 

 nings believed this to be true. He re- 

 garded the asexually produced descend- 

 ants of a single Paramecium as consti- 

 tuting a pitre line, which selection coiild 

 not change, and recognized several diff- 

 ferent pure lines isolated from his mixed 

 cultures, each \'arying about a different 

 average of size. Within each pure line 

 he found selection ineffective. 



IN ASEXUAL REPRODUCTION. 



But considerable doubt is raised 

 concerning the soundness of these con- 

 clusions by the subsequent work of 

 Calkins and Gregory^ who find that 

 from a single "pure line" as defined by 

 Jennings subordinate lines may be 

 isolated which differ in mean size from 

 each other more than the pure lines 

 which Jennings supposed to be geneti- 

 cally different. If this is so, it must be 

 Ijossible for genetic differences to arise 

 in the course of asexual reproduction, 

 which on the pure line hypothesis is im- 

 jjossible; and if such genetic differ- 

 ences do arise, they afford material for 

 selection to act upon with effectiveness, 

 even within the supposed pure line. 



Further evidence that racial differ- 

 ences may originate within an asexually 

 ])roduced and so genetically "])vire line" 

 is afforded by horticultural varieties of 

 plants proi)agate(l asexually. Examijles 

 are found among a]j]jles and j^otatocs. 

 An orchard variety which has endured 

 for any considerable length of time is 

 apt to develop sub-varieties. The same 

 is true of varieties of potato which have 

 enjoyed wide and long continued popu- 

 larity. The Williams apple, widely 

 cultivated in the eastern United States, 

 occurs in two distinct strains, one con- 

 s])icuously striped, the other nearly 



solid red. The Baldwin apple which 

 has been in cultivation for over a cen- 

 tury has developed sub-varieties differ- 

 ing in shape. The Early Rose potato, 

 formerly the favorite early variety in 

 the United States, developed sulj-\-a- 

 rieties, some of which were more 

 elongated in shape than others. 



So common are changes of this sort 

 within asexually propagated \-arietics 

 that botanists have given them a special 

 name, that of hud variations. East, 

 from his studies of bud variation among 

 potatoes, concluded them to be ordinarily 

 loss variations, or retrogressive ^■aria- 

 tions due to the loss of some normal 

 racial character, as are most of the 

 germinal variations which arise as sports 

 in the course of sexual reproduction 

 among domesticated animals and culti- 

 vated plants. If so, we are led to con- 

 clude not only that variations may 

 occur within a "pure line" asexually 

 produced, but also that these variations 

 arc of the same nature as those which 

 arise in the course of sexual reproduc- 

 tion. There can be no question of the 

 effectiveness of selection in cither cate- 

 gory of cases. 



The Hagedoorns think that evidence 

 in favor of the pure line hypothesis is 

 afforded by Vilmorin's \'arieties of 

 wheat bred each from a single plant. 

 They figure four heads of wheat selected 

 each as typical of those ]jroduced by the 

 ancestral plant 50 years ago, from 

 which a j^urc variety has descended. 

 Beside these they figure a typical head 

 of each of the derived varieties at the 

 present time. A strong resemblance 

 exists in each case between the head of 

 50 years ago and the ]jresent day repre- 

 sentati\'e of the same variety, though 

 this of cour.sc does not amount to 

 identity. We are, however, told that 

 "As can easily be seen from the photo- 

 gra])h, all these generations of selection 

 have not changed any one of the varie- 

 ties one little l)it." 



IvVIDENCli NOT COMPLETE. 



The ]jhotographs aftord a rather small 

 basis for so swee])ing a conclu.sion. We 

 are not shown the entire ])lant of 50 

 years ago beside that of today ; we know 



^Journal of Ex]KTinu'ntal Zoology, Vol. 1,S, \o. 4, Nov. 191.?. 



