January, 1938 



EVOLUTION 



Page Seven 



acters which are peculiar to certain mutants also tend to 

 persist in subsequent mutants from these. 



Mr. McKinney and other investigators have found that 

 certain types of virus mutation are more likely to occur 

 under certain conditions, for example, under the influence 

 of high temperatures. 



The virus cannot be identified under the most powerful 

 microscope. It will pass through fine filters and retain its 

 disease producing character. To identify a virus, experi- 

 menters are driven to observe its effects on the plants it 

 attacks. The virus multiplies in the plant, but no experi- 

 menter has been able to cause it to multiply outside the 

 plant. Some viruses can exist in more than one species ot 

 plant, and two viruses causing identical symptoms in one 

 species may be distinguished by differences in the symp- 

 toms on another species. 



Several investigators have experimented with the crystal- 



like virus protein. They have been able to dissolve this 

 and cause all the known signs of the disease in tobacco 

 and other suitable plants which were inoculated with the 

 solution. Some workers have considered the crystallization 

 test as proof that the virus has been isolated as a pure 

 chemical substance. However, this test alone is not ac- 

 cepted as positive proof because there is still a possibility 

 that the virus may be present as an impurity. 



Although Mr. McKinney considers that the "virus and 

 its mutants doubtless reflect a series of closely related 

 compounds which function essentially as genes," he takes 

 the position that this conclusion is still consistent with the 

 idea — held by some investigators — that the virus represents 

 a complex protein molecule. Some investigators consider 

 the genes of plants and animals to be proteins. The virus 

 particles may represent free parasitic genes of a primitive 

 type, but nevertheless similar to the genes which occur 

 normally in plants and animals. 



The Glacial Period 



By J. HARLEN BRETZ 

 Professor of Geology, University of Chicago 



KINDLY visualize a brick. Chicago is built almost 

 entirely of such bricks. It is also one of the greatest 

 brick producers. Brick is made from clay. Chicago bricks 

 from Chicago clay. That's easy to understand. But how 

 was the clay made? Someone answers "Why, the clay 

 was here in the first place." 



Chicago uses vast quantities of crushed rock in concrete 

 work. The rock is limestone, blasted out of immense 

 quarries in and adjacent to the city. These quarries are 

 located where the limestone is close to the surface. In 

 most places it is buried beneath 50 to 100 or more feet 

 of the clay. Now if the clay was here in the first place, 

 the limestone either was put under it afterward (which is 

 absurd), or was made before "the first place" (which is 

 equally absurd). The limestone is beneath the clay and 

 therefore is surely older than the clay. What you meant 

 by "the first place" was "before Chicago was started." 



Chicago uses vast quantities of sand in its building 

 operations and in raising its railroad grades above street 

 grades. Most of this sand comes from the dunes in 

 northern Indiana, along the shore of Lake Michigan. 

 "Where did the dunes get it?" That's easier to answer 

 than "where did the clay come from." Any visitor at the 

 Dunes sees almost at a glance that the dune sand is simply 

 the beach sand, swept up from the shore of the lake by 

 the wind. 



The brick clay, the limestone and the dune sand all were 

 here before Chicago's history began. Only the dune sand 

 is still accumulating, and its method of origin therefore 

 perfectly clear. How was the clay made? 



If it were necessary to witness the events of the history 

 of the earth to know of their occurrence, we would know 

 precious little about the subject. The old earth has been a 

 long, long, long time in the making, and Man as an in- 

 telligent observer has been writing records on stone, parch- 

 ment and paper for only a few thousands of years. 



But, happily, the processes by which the Earth has been 

 brought to its present condition have left indelible, unmis- 

 takable records of their own. Graven on stone, too, but 

 the hieroglyphs are wholly different from any that men 

 ever designed. When we learn to read them, we can 



answer that question "How was the Chicago clay made?" 

 Let's have a look at that clay. 



We are standing now on the brink of one of the big 

 Chicago pits. The steam shovels have dug deeply to the 

 bed-rock, the limestone. Look at the surface of that lime- 

 stone from which the clay has been removed. It is as 

 smooth as a floor. And look at those parallel grooves and 

 furrows across it, all directed northeast-southwest. Looks 

 as though some great planing machine has been run across 

 it. If we go down into the pit and scrutinize the surface 

 of the limestone more carefully, we find millions of tiny 

 scratches, as fine as though made with the point of a needle, 

 and all parallel to the course of the grooves and furrows. 

 Whatever made the surface of the limestone smooth, what- 

 ever made those grooves and furrows and tiny scratches — 

 moved across the bedrock surface either from the northeast 

 or from the southwest. 



A Giant Planing Machine 



While we are down in the pit, let's look at the clay 

 more carefully. It isn't all clay! It has many stones in it, 

 some three or four feet in diameter. And they have the 

 same sort of markings on them as the bedrock! Except that 

 they point in various directions on the same boulder. 



We may assume that whatever made the markings on the 

 surface of the bedrock, also made the markings on the 

 stones in the clay. Then the clay which contains the 

 boulders and which lies on the scored surface of the lime- 

 stone, was made at the time the markings were made. 

 And isn't it perfectly logical to think that the scratched 

 boulders and stones were the tools used by our hypothetical 

 planing machine in smoothing, grooving and scratching the 

 surface of the underlying limestone? 



Here's another significant thing. Many of these stones 

 are not of limestone. Here's one of granite! Another 

 of sandstone! A third one of quartzite! Here's another 

 granite, different from the first! We shortly identify 

 twenty or more different kinds of rock, none of them lime- 

 stones. Now where did they come from? There's no 

 granite under Chicago for half a mile down, as several 

 deep wells in town tell us. Therefore the boulders of 



