92 BOTANICAL GAZETTE [july 



a) The idea that Linnean species form a natural system, 

 i. Belief in the constancy of Linnean species. 

 2. Belief in the variability of Linnean species. 



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B. The period in which proof was given of the independent transmission and chance 



recombination of characters. 



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rguments 



which there is room for a difference of opinion: (i) that all characters obey 

 the Mendelian law of heredity, (2) that acquired characters are never trans- 

 mitted, (3) that homozygotes are absolutely constant in successive generations, 



(4) that there has been no proof of variation independent of crossing, and 



(5) that the variations observed after crossing are sufficient to account for 

 evolution. 



The first point is taken for granted, although there are certain to be many 

 objections to it raised. Even the most ardent Mendelians have only gone so 

 far as to assert the generality of Mendel's law in the sense that the transmission 

 of many characters is controlled by some mechanism that is widespread in the 

 animal and plant kingdoms, probably the chance apportionment of the chro- 

 mosomes to either of the two daughter cells at the reduction division. The 

 probability that there are many cases where such a mechanism is replaced by 

 others can hardly be denied, though Jennings' recent investigations on Para- 

 mecium indicate a generality of segregation hitherto unsuspected. Further- 

 more, even in cases where Mendel's law might be expected to be valid from 

 other considerations, as in the Oenotheras, there appear to be independent 

 or subsidiary laws at work which modify the results. 



Perhaps most biologists admit that the inheritance of acquired characters 

 has never been proved experimentally; on the other hand, it may be that it is 

 impossible to prove that the hour hand moves in an experiment covering a 

 second of time, as Butler very aptly puts it. Is it not justifiable, therefore, 

 for experimentalists to assume non-inheritance of acquired characters as a 

 practical working hypothesis for experimental biology, and just as permissible 

 for evolutionists to assume their transmission over long periods of time, pro- 

 vided it is granted that this is a mere assumption ? Lotsy applies the negative 

 conclusions of experimental biology to evolution, a method that is always open 

 to criticism. 



The contention that homozygotes are absolutely constant in all succeed- 

 ing generations is also disconcerting. The conclusion is drawn largely from 

 Johannsen's work, yet Johaxnsen believes in mutations. But even leaving 

 out of account the sudden changes that have appeared in material apparently 

 homozygous, there are those who believe that the pure line work does not 

 show constancy in succeeding generations. Castle, for example, who does 

 not come to a conclusion without due consideration, believes that selection 



