PRESENT CRISIS IN EVOLUTIONARY THOUGHT 21 



work might be trusted to supply the deficiency. It has not 

 done so, and the significance of the negative evidence can 

 no longer be denied. . . . 



"If species have a common origin where did they pick up 

 the ingredients which produce this sexual incompatibility? Al- 

 most certainly it is a variation in which something has been 

 added. We have come to see that variations can very com- 

 monly — I do not say always — be distinguished as positive and 

 negative. . . . Now we have no difficulty in finding evidence 

 of variation by loss, but variations by addition are rarities, 

 even if there are any such which must be so accounted. The 

 variations to which interspecific sterility is due are obviously 

 variations in which something is apparently added to the stock 

 of ingredients. It is one of the common experiences of the 

 breeder that when a hybrid is partially sterile, and from it any 

 fertile offspring can be obtained, the sterility, once lost, disap- 

 pears. This has been the history of many, perhaps most, of 

 our cultivated plants of hybrid origin. 



"The production of an indubitably sterile hybrid from com- 

 pletely fertile parents which has arisen under critical observa- 

 tion is the event for which we wait. Until this event is wit- 

 nessed, our knowledge of evolution is incomplete in a vital 

 respect. From time to time such an observation is published, 

 but none has yet survived criticism." {Science, Jan. 20, 1922, 

 pp. 58, 59.) 



But what of the chromosomal mutant? For our knowledge 

 of this type of mutation we are largely indebted to Blakeslee's 

 researches and experiments on the^Jimson weed {Datura stra- 

 monium). According to Blakeslee, chromosomal mutants re- 

 sult from duplication, or from reduction, of the chromosomes, 

 and they are classified as balanced or unbalanced types 

 according as all, or only some, of the chromosomal link- 

 age-groups are similarly doubled or reduced. If only one of 

 the homologous chromosomes of a synaptic pair is doubled, 

 the mutant is termed a triploid form. It is balanced when 

 one homologous chromosome is doubled in every synaptic 



