Varietäten, Descendenz, Hybriden. 583 



the one sought, and is quickly gotten by subtracting the surface 

 for the relationship between individual parents and their ofifspring 

 from the ascendant correlation surface just described. 



Jongmans. 



Harris, J. A., The biometric proof of the pure line 

 theory. (The American NaturaHst. XLV. p. 346—363. 1911.) 



The author publishes following summary and conclusions: 



ßy the genotype theory of Johannsen one understands the 

 following propositions: 



An apparentl}^ uniform population or phaenotype is generally 

 not homogeneous, but is composed of a large number of diffe- 

 rentiated types, which are to be designated — within limitations to 

 be laid down immediately — as genotypes. 



Externally, the genotype can not be distinguished from the 

 phaenotype. Both may have normal Variation curves, but while 

 that of the phaenotype may by proper selection be broken up into 

 constituent genotj^pes, the Variation curve of the genotype can not 

 be modified by selection. In Short, the genotype is from the stand- 

 point of heredity a rigid unit. All individuals belonging to the 

 same genotype have the same potencies as parenis. Only dis- 

 continuous segregations or transformations — mutations — may 

 modify them.. 



The keystone of the pure line arch is the proposition that se- 

 lection is ineffective except as a means of separating already existing 

 genotypes. If this keystone-proposition be not sound the whole 

 structure of the theory crumbles. 



The propositions of the genotype theory are such that scientific 

 proof or disproof is rendered particularly dißicult. By theory selec- 

 tion can not effect a change in a pure line; by a slippery process 

 of reasoning in a circle any results attained bj^ selection are at 

 once discredited by the assertion that the original material was 

 impure. If, on the contrary, any selection experiment is ineffectual 

 it is by some process of reasoning quite incomprehensible to sorae 

 of US, at once chalked up to the credit of the new theory. If herita- 

 ble differences appear within a pure line known to be so, these 

 results are also discredited by the assertion that the observed change 

 is a mutation or has been produced by the action of the environment. 



The actual experimental data upon which the genotype theory 

 rests are as yet few. Johannsen 's conclusions for beans depend 

 chiefly upon the offspring of only nineteen seeds, and so far as the 

 author is aware no other investigator has confirmed his results on 

 Phaseolus. Hanel had only twenty-six original Hydra, and Pear- 

 son's analysis of his data with more adequate methods than he 

 used, evidences against rather than for the genotype theory. Jen- 

 nings gives us the records of only six selection experiments invol- 

 ving altogether only a few actually selected Parainecia. Considering 

 the large environmental and growth factors, his conclusions cannot 

 be considered as beyond question. The work of Pearl and Surface 

 with poultry and maize seems to the author to have no critical be- 

 aring on the pure line problem. This is also true of numbers of 

 other smaller experiments which can not be cited. 



If one turns from the strictly pure line side of the problem to 

 the more general questions of the „something" in the germ plasm 

 which determines in large degree the somatic characters of the in- 



