“If species have a common origin where did they pick up the ingredients which produce this sexual incompatibility? Almost certainly it is a variation in which something has been added. We have come to see that variations can very commonly—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, disappears. This has been the history of many, perhaps most, of our cultivated plants of hybrid origin. “The production of an indubitably sterile hybrid from completely fertile parents which has arisen under critical observation is the event for which we wait. Until this event is witnessed, 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 stramonium_). According to Blakeslee, chromosomal mutants result 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 linkage-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 pair, but if one or more chromosomes be added to, or subtracted from, this balanced triploid complex, the mutant is termed an unbalanced triploid. When all the chromosomes of the normal diploid complex are uniformly doubled, we have a balanced _tetraploid_ race. The subtraction or addition of one or more chromosomes in the case of a balanced tetraploid complex renders it an unbalanced tetraploid mutant. The retention in somatic cells of the haploid number of chromosomes characteristic of gametes and gametophytes gives a balanced _haploid_ mutant, from which hitherto no unbalanced haploids have been obtained. The normal diploid type and the balanced tetraploid type are said to constitute an _even_ balance, while balanced triploids and haploids constitute an _odd_ balance. The odd balances and all the unbalanced mutants are largely sterile. Thus, for example, more than 80% of the pollen of the haploid mutant is bad. “The normal Jimson Weed,” says Blakeslee, “is diploid (2n) with a total of 24 chromosomes in somatic cells. In previous papers the finding of