142 GENETICS 



conclusions of the investigators are not in entire har- 

 mony. There is no doubt that in most cases selection 

 within a parthenogenetic line is futile although Banta's 

 long continued observations upon daphnids seem to fur- 

 nish evidence of an opposite kind. Particular weight 

 should be given to this work because it presents one of 

 the longest pure lines that ever passed under the seeing 

 eye of a scientist. In some of his lines there have been 

 450 generations (1921) forming an unbroken line 

 extending over 10 years' time. If this pedigree were 

 translated into human generations of 30 years each it 

 would make a period of 13,500 years and would run 

 back over 100 centuries B. C. long before the very 

 beginnings of human history. There is no doubt that 

 many experiments in selection cannot be considered 

 decisive because they concern altogether too few gen- 

 erations, as compared with the time that has been at the 

 disposal of nature in accomplishing evolutionary 

 change. 



D. HOMOZYGOUS CEOSSES 



It is very difficult to find instances among animals 

 and plants where two individuals are homozygous in all 

 particulars. The nearest approach is "identical twins" 

 which arise from a single fertilized egg and consequently 

 are more nearly germinally alike, and can never cross since 

 they are always of the same sex. 



It is useful, nevertheless, to consider pure lines result- 

 ing from homozygous crosses when limited to a single 

 character rather than to individuals, for of this con- 

 dition there are numberless instances. 



