378 



SCIENCE 



[N. S. Vol. LI. No. 1320 



while those that show a reaction with the " d " 

 race of Mucor V and not with its " c " race 

 are placed in the left column. In no case has 

 the position of any of the races been deter- 

 mined by less than two positive reactions. 

 Any race in the left column is theoretically 

 capable of showing a reaction with any in the 

 right column, while incapable of reacting with 

 those in its own group and vice versa. Cer- 

 tain combinations, however, react with greater 

 difficulty than others. It is obvious that in 

 these two colimins we have represented the 

 two opposite sexes, male and female. There 

 seemed, however, no way of determining 

 which is to be considered male and which 

 female since their gametes are typically equal 

 in size. 



In making the diagram, it was observed 

 that of those species, in which there was 

 evident a greater vegetative vigor of one 

 sexual race over the other, the more vigorous 

 race was always in the left-hand column. All 

 those in the left-hand column were accord- 

 ingly called plus and those in the other 

 column minus despite the fact that in many 

 species no vegetative difference between the 

 sexes could be established. The most striking 

 example of a difference in vegetative vigor is 

 that of Mucor III shown in Fig 58 (1). In a 

 considerable number of races in several differ- 

 ent species, however, I have found that the 

 plus race is not invariably more vigorous than 

 the minus when a difference in vegetative 

 vigor is observed, judging vigor by former 

 criteria; but this fact does not detract in the 

 least from the evidence that in the plus and 

 minus races we have the two sexes repre- 

 sented. 



The " imperfect hybridization " reaction is 

 of convenience in determining the sex of un- 

 mated races. Thus when the diagram (Fig. 

 2) was made, a race of Circinella umhellata 

 obtained from a curbstone in the shadow of 

 Harvard University, by reacting with Mucor 

 V plus and failing to react with Mucor V 

 minus, was found to be a minus race and is so 

 listed in the diagram. Later a race was ob- 

 tained from a substratum sent by a mission- 

 ary from China and was discovered to be its 



plus mate. It was a relatively easy matter 

 then to obtain the zygospores by uniting these 

 opposite races under suitable nutrient and 

 temperature conditions. The last species in 

 the minus column was found in 1903 in a 

 culture of rat's whiskers gathered on an island 

 in the Caribbean Sea. Perhaps somewhere, 

 uiider some spreading palm, from India's coral 

 strand, its mate is waiting; and another good 

 missionary may help in spreading the gospel 

 of a " form new to science." 



I said a moment ago that it is theoretically 

 possible to obtain an " imperfect hybridiza- 

 tion " reaction between the sexual race of a 

 given species and the opposite sexual race of 

 any other species. In practise it has not 

 always proved easy of accomplishment. Much 

 depends upon the environmental factors such 

 as the kind of nutrient — more, however, upon 

 the sexual vigor of the reacting races. A 

 race may react with the opposite sex of an- 

 other species under temperature or nutrient 

 conditions which will not allow it to form 

 zygospores with its normal mate in its own 

 species. Thus Cunninghamella echinulata 

 will readily give " imperfect hybridization " 

 reactions with species of the genus Mucor at 

 temperatures below 20° 0. but will not itself 

 form zygospores at so low a temperature, 

 while some species of the genus Mucor are 

 weak in reaction when contrasted inter se. 

 The vigor of the reaction, therefore, has no 

 apparent connection with the taxonomic rela- 

 tionships of the forms involved. Cunning- 

 hamella it may be remembered is so distantly 

 related to the genus Mucor that it was origin- 

 ally described as a Hyphomyeete and assumed 

 to belong to a group of fungi imrelated to 

 the mucors. 



Saito and N'aganishi (13) report obtaining 

 true hybrid zygospores between different 

 Mucor species. They admit, however, that 

 the species in question are very closely re- 

 lated. I have found, between what I have 

 called the opposite sexes of a single species, 

 differences sufficiently marked to be worthy of 

 description as distinct si)ecies according to 

 Bainier who has been one of the mycologists 

 most prolific in fathering Mucor species. I, 



