420 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1930 



And DrosopJiila is not exceptional. Let us take a small water crus- 

 tacean, one of the Daphnids. Banta has reared lines of these in 

 captivity and examined the progeny daily- In one line of Moina 

 ■macrocopa^ carried parthenogenetically, a dominant mutation has 

 occurred, on the average, at least once in 50 generations, but many 

 more recessive mutations have occurred and been phaenotypically un- 

 expressed. Now the number of Daphnids, which crowd any suitable 

 pond in both hemispheres during each spring and autumn, is beyond 

 conception. For a single circular pond a hundred feet in diameter 

 may well contain during the season many million Daphnids, if one is 

 allowed to the cubic centimeter. The total of mutations that occur 

 in one year in Moina macrocopa must be inconceivably great. 



Certain of the lower forms are mutating even more strikingly. At 

 least such would seem to be the case if the remarkable variations 

 shown by Leonian in the fungus, Fusariwn, may be regarded (as 

 seems most probable) as mutations. Here scores of strains arise, in 

 but a few years, even in a uniform culture medium, and perpetuate 

 themselves. The strains vary in their rate of growth, pigment forma- 

 tion, type of fruiting, kind of spores, and reactions toward tempera- 

 ture, acids, dyes, and toxic substances. Apparently such mutation is 

 going on all the time in nature. 



As we consider these best known cases of mutation and realize 

 that all of the countless chromosomes and genes are undergoing 

 occasional change we are appalled by the universality of mutation 

 and are caused to wonder how any species remains constant in nature 

 to the extent that it is possible for a second naturalist, 50 years later, 

 to identify in nature the species already described; we are less sur- 

 prised that the reviser of a genus a generation or two later will find 

 twice as many species as his predecessor. We gain a lot of sympathy 

 for the much abused species splitter who, observing nature without 

 the restriction of tradition, finds vastly more species than had been 

 previously described by his predecessors. 



Organisms seem to be producing mutations at an inconceivably 

 rapid rate, in infinite quantity The wonder is that there are such 

 things as species. One is led to inquire if, in describing species, tax- 

 onomists are not merely inventing transient, evanescent categories. 



Such a conclusion is unjustified. Every taxonomist will tell you 

 that the things he describes and others have described before him are 

 real entities. If I am studying thrips and wish to secure a species 

 described 50 years ago as living in a certain composite plant in eastern 

 Russia, then if I go to the designated locality and look in the desig- 

 nated species of flower I will find the species with all the characters 

 described 50 or 100 thrips generations ago. How is such an experi- 

 ence in constancy to be harmonized with universal mutation? This 

 is perhaps the heart of the problem of evolution. 



