true as regards the disintegration products of 

 radioactive elements, but recent discoveries 

 bearing on the nature and distribution of cosmic 

 rays have changed the situation considerably. 

 We now know that photons and high -energy 

 charged particles reach the earth in large quan- 

 tities. These particles by their properties and 

 behavior are very similar to the radiations 

 produced by X rays and to the emanations of 

 radioactive bodies; consequently, there is no 

 reason for assuming that their effect on nuclei 

 will be any different. However, the difference 

 is that X rays, under experimental conditions, 

 fall in a compact and continuous beam upon the 

 chromosomes, whereas cosmic rays bombard 

 them in a diffused fashion with separate and 

 infinitely tiny particles. If we take as a meas- 

 ure of the activity of X rays the amount of 

 energy used for tobacco by Goodspeed and Olson 

 (1928), we shall see that their action is more 

 intense than that of cosmic rays at sea level. 

 The majority of doses used to induce mutations 

 is 1500 times stronger than the action of cosmic 

 rays. However, recent experiments have shown 

 that the ionization produced by cosmic rays falls 

 rapidly as it passes through the atmosphere of 

 the earth and that it increases with altitude. 

 Thus, at an altitude of 11, 000 feet the intensity 

 of radiation increases almost three -fold; at an 

 altitude of 22, 000 feet, 10-fold; at an altitude of 

 62, 000 feet the ionization is 83 times as great 

 as at sea level. In the stratosphere the intensity 

 of cosmic rays is 150 times as great. This 

 radiation does not consist of a uniform stream 

 of equally charged particles; their composition 

 also changes with altitude. In addition, from 

 the author's point of view, the phenomena known 

 as explosions or showers of cosmic rays are 

 interesting in that they represent beams of 

 ionized particles, which upon striking, say, a 

 mass of sporocytes, can deliver direct blows 

 to the various chromosomes. 



Compton and Bennett^ believe that the density 

 of these beams increases with altitude more 

 rapidly than the intensity of the whole beam of 

 cosmic rays. Compton has demonstrated that 

 showers on Mount Evans (6900 feet at 50° 58" 

 north latitude) liberate more than 3 x 10 ions. 

 The same ionization on the same latitude was 

 applied to tobacco by Goodspeed and Olson 

 [1928] in their experiments. This brings us to 

 the conclusion that if modifications of the nuc- 

 leus and consequent mutations arise due to the 

 action of cosmic rays, the frequency of their 

 occurrence must change with the altitude and 

 location of the plants. At sea level few muta- 

 tions are observed, but their number rises with 

 altitude. It should be pointed out, however, that 

 in high altitudes the intensity of ionization is 

 affected by the latitude: measurements per- 

 formed at 22, 000 feet in Panama (20° N) and in 



^Reference given in article by Thomas (1936) cited 

 above. 



Peru (4° S) are 29% lower than those made in 

 California (41° N) and 38% lower than in Wash- 

 ington (54° N). On the other hand, the altitude 

 at which plants can grow falls rapidly as the 

 latitude increases. Consequently, in order to 

 clarify the question of the influence of altitude 

 above sea level on the changeability of species 

 and intensity of cosmic radiation, observations 

 must be made at various points on the surface 

 of the earth. 



Certain preliminary data permit us to believe 

 that such an influence exists. Vavilov's^ data 

 on the geographic distribution of varieties of 

 the most important grain plants are particularly 

 interesting in this respect. He established that 

 the greatest variations in species and varieties 

 of grain crops are found in mountains of the 

 tropics and moderately warm areas. Thus, he 

 noted 60 varieties of Triticum vulgare in 

 Afghanistan, 52 in Persia, 46 in Baluchistan, 

 but only 32 in India, 31 in Mongolia, and 12 in 

 Italy. Vavilov determined seven or eight cen- 

 ters where agricultural plants originated. 

 Nearly all these points are located in high 

 mountains or plateaus not more than 40° from 

 the equator. There also can be no doubt that 

 the flora of mountain areas of the tropics and 

 moderately warm countries is far more varied 

 than in the lowlands. Standly points out that 

 the flora of little Costa Rica has more than 6000 

 species of vascular plants, i. e. , nearly as 

 many as in the whole southeastern United States, 

 although Costa Rica is only half the size of 

 Florida and has many mountainous areas that 

 have not yet been explored botanically. The 

 mountains of Costa Rica rise to 11, 000 feet and 

 more than half of the country lies above 3000 

 feet. Observations indicate that the plant inhab- 

 itants of these tropical mountain areas are very 

 variable; however, the question remains to 

 what extent the richness of varieties can be 

 explained by this means. Costa Rica is an 

 interesting example since it is a young mountain- 

 ous area of the late Eocene and Oligocene peri- 

 ods, but a large percentage of its endemic flora 

 is such that we cannot assume that it is due to 

 the spread of species from the north to the 

 south. 



The author's idea [Thomas, 1935] is neatly 

 confirmed by the genus Primula . Although the 

 section Vernales is widely distributed in Europe 

 and in eastern Asia, chiefly in the lowlands, it 

 contains only 19 species; whereas numerous 

 other sections inhabiting mountainous areas are 

 much richer in species and endemic forms 

 (e.g. , Petiolares has 66 species, Nivales has 

 61, etc.). There is a total of some 330 species 

 of mountain primulas. Almost all the alpine 

 genera of Primulaceae are richer in species 

 than the genera of the valleys. 



Jepson* considers it proven that the greatest 

 profusion of new varieties is associated with 

 gradual slopes of mountains. 



112 



