EXPERIMENTAL EVOLUTION, 129 



It may be a cytological modification, but in the great majority of cases 

 it is chromosomal. It has been m-ged that, when chromosomal, it may 

 be due to a change in the nature of a gene without a change in its iden- 

 tity. But a change in a gene is, for the geneticist, a new gene, whether 

 it occupies the place of a pre-existing gene or not. It troubles some ex- 

 perimentalists that, through selection, a change in the race can be so 

 soon effective; but the change had already occurred before they began 

 their effective selection; they were merely working with a germinal 

 change that had occurred before the phenotypic change which they were 

 "selecting." In a species or biotype that is invariable we may be 

 surprised if we make progress in a given direction; but in a biotype 

 that is already showing mutations we may expect further mutations in 

 every generation. Of course, size of the phenotypic change induced 

 in the mutation is never to the point ; sometimes two or more changes 

 in genes induce no visible, phenot^^^ic change. 



SEX IN MUCORS. 



Work on the sexuality of the mucors has been continued by Dr. 

 Blakeslee. He has accumulated a considerable body of new data on 

 the results of testing the sexual interactions of dicecious species. In 15 

 different species, representing 9 different genera, over 1,600 individual 

 races have been tested in pairs for the production of zygospores. Nearly 

 10,000 combinations have thus been made, with the result that each 

 race tested has been shown by the presence or absence of zygospore for- 

 mation to belong to the plus ( +) or minus ( — ) sex or to show no sexual 

 reaction in any combination. No sex intergrades have been discovered. 

 Different races have different strengths of sexual vigor, as shown 

 by the intensities of zygospore formation. The "neutral " races, which 

 have not as yet shown zygospores in any combination, may be the low 

 extremes of a graded series into which the sexually active races can be 

 arranged, based on the abundance of zygospores which they form in 

 different combinations. We have just completed a series of tests of 

 sexual reactions between races of different species and have thus made 

 a total of over 2,000 combinations. "Imperfect hybridization" has 

 taken place only when the races grown in contact belonged to the oppo- 

 site sexes, minus ( — ) and plus ( + )• The "imperfect hybridization" 

 work confirms the conclusion dra\\ai from the tests between races of 

 the same species, viz, that a stricter sexual dimorphism is present in 

 the dioecious mucors than in dioecious species of higher plants. It is 

 suggested that this difference may be connected with the fact that in 

 the mucors we are dealing with gametophytes, while in higher plants 

 we are dealing with sporophytes. 



Dr. Blakeslee has pointed out, in his recent vice-presidential address 

 before Section G of the American Association for the Advancement of 

 Science, that as a criterion of sex the relative size of the two uniting 

 gametes (the larger being called female and the smaller male) does not 



