No. 508] SHORTER ARTICLES AND CORRESPONDENCE 241 



PUKE STRAINS AS- ARTIFACTS OF BREEDING 



Students of the minute anatomy of plant or animal tissues 

 are on their guard against artifacts. Chemical reagents often 

 contract or coagulate the protoplasm or cause the precipitation 

 of granules or crystals. The artificial results of the methods of 

 preparation have to be distinguished from the normal structures 

 of the protoplasm. Precautions are equally necessary in the 

 study of evolution and heredity, to avoid mistaking artificial 

 products of breeding for typical conditions. 



Close-bred, uniform groups of plants and animals form the 

 basis of the idea of "pure strains," "elementary species" or 

 "biotypes." If the descendants of the same parents appear 

 sufficiently alike the strains are said to be "pure." General 

 inferences regarding heredity and evolution are being based 

 on the assumption that this uniform "purity" represents a 

 natural condition. Yet there can be no doubt that the methods 

 used in maintaining and testing the purity of strains are calcu- 

 lated to produce an artificial uniformity of characters, commonly 

 accepted as the proof of purity. 



If the habits of a plant will permit, the readiest method oi 

 securing uniform progeny is by vegetative propagation. Never- 

 theless the idea of pure strains is oot usually connected with 

 vegetative varieties, for it is recognized that uniformity lasts 

 only while vegetative propagation is continued. As soon as 

 seeds and seedlings are grown the natural individual diversity 

 reappears. The vegetative propagation only conceals t w in- 

 herent diversity by devices that avoid the production of seedlings. 



The uniformity of groups of seedling plants is of the s.-ime 

 artificial nature as the uniformity of vegetative varieties. ar- 

 ticular methods of reproduction are necessary t«» secure 1,1 

 seedlings— methods which may not be essentially °& en ^ 

 those represented by cuttings or offshoots. 1 hints _muu ^ m 

 buds or cuttings have only one parent, and the same is ^! u(M> 

 seedlings produced by self-fertilization. It is only \\ ion e,m- 

 jugation is restricted to cells of the same individual or o. e ose. 

 related individuals that uniform seedlings are obtained. The 

 external formalities of conjugation are preserved, but without 

 the essential diversity of descent which gives conjugation a 

 physiological significance. 



Tlle „„ i f„™ity of varieties of wheat and other strut y self- 

 fertilized plants depends as closely upon self-ferfhzat,on as 



