248 Biology in America 



behave furthermore as definite, fixed characters in inherit- 

 ance, Mendelizing when crossed with the original stock. 



It is well known that the X-rays and those of radium and 

 related substances exercise a profound influence on living 

 tissue. In general the influence of such agents has been de- 

 structive, inducing abnormal developments of various sorts. 

 The careless use of X-rays has been the cause in some in- 

 stances of cancer, while on the other hand their use, and es- 

 pecially that of radium rays has been advocated as a cure for 

 this disease. Unfortunately the results in this direction are 

 as yet unsatisfactory. 



In some instances it has been possible to produce changes 

 in the developing organism without any apparent injury to 

 it. One of the prettiest experiments of this sort is that of 

 Gager, who exposed the pollen of one of the evening prim- 

 roses to radium rays, and obtained a few individuals with 

 thick, leathery leaves from ovules fertilized by this pollen, 

 which character reappeared in their offspring. That such 

 radiations produce profound effects upon the structure of the 

 cell is shown by exposing growing root tips to their influence, 

 the mitotic figures in the dividing cells being greatly distorted 

 thereby. 



The probable influence of environment upon animal form 

 and color is very clearly shown in the extensive collections 

 of birds and mammals made by American ornithologists and 

 mammalogists, especially those of the U. S. Biological Sur- 

 vey, in which an almost endless series of gradations may be 

 seen between the various " geographic races" of one ''spe- 

 cies" 5 from different parts of North America. These varia- 

 tions are so great in some cases that specimens from extremes 

 of the geographic range would not be recognized as members 

 of the same species, did not a complete series of intergrades 

 exist. 



If one compares a mammal, a field mouse let us say, from 

 Florida or tropical Mexico with its nearest relative from 

 Labrador or Alaska, he will find the legs, ears and tail of 

 the former a trifle longer, and the fur a little lighter than 

 the same parts of its northern cousin. Is it mere accident 

 that the northern mouse is more warmly clad, and has ap- 

 pendages which are shorter, and consequently less liable to 

 freeze, than the southern one? Or is it a question of sur- 

 vival, has Nature selected those individuals best adapted to 

 the regions where they live? Or yet again, is this a case of 

 the influence of environment, and if so, has the latter di- 



5 According to some writers these ' ' geographic races ' ' are accorded 

 the status of ' ' species. ' ' The question of terminology does not how- 

 ever influence the facts in the case. 



