388 THE POPULAR SCIENCE MONTHLY 



NATUEAL SELECTION 



By Professor T. D. A. COCKERELL 



UNIVERSITY OF COLORADO 



THE lowest forms of life show the greatest stability in their specific 

 characters. Any one who will examine a little pond water under 

 the microscope will see numerous minute Protozoa belonging to different 

 families, genera and species. Close study brings out the fact that 

 although we regard these as very low types, they are complicated little 

 animals, with remarkable characters. When we look at lists of these 

 creatures, based on collections from different parts of the world, we are 

 astonished to find that most of the species are the same, no matter how 

 remote the localities. When identifications were based merely on com- 

 parisons of descriptions and figures, we suspected that the alleged wide 

 distribution of some of these fresh-water Protozoa might be due to mis- 

 takes. In recent years, however, protozoologists have frequently trav- 

 eled, and Dr. Penard, the greatest authority on rhizopods, has been 

 able to determine by study on the spot the identity of Eocky Mountain 

 forms with those of Switzerland. Even more remarkable are the results 

 of Dr. Edmondson, who has visited Tahiti, high altitudes in the Eocky 

 Mountains, and various places in the western central states, with the 

 result of entirely confirming the opinion that most species of fresh-water 

 Protozoa are spread over the world, almost without regard to climate 

 or altitude. 1 



These studies and reports, however, have been based on the species 

 as generally understood. These species are not entirely uniform, but 

 consist of groups of minor races, which also appear to have constant 

 characters and to be of general distribution. Dr. Penard told me that 

 he could greatly increase the list of " species " of rhizopods were he to 

 describe as distinct all the apparently constant forms which he had 

 learned to recognize, and which, so far as he knew, did not conjugate 

 one with the other. He did not describe and name them because their 

 separation required such critical comparisons and familiarity with the 

 subject that very few naturalists would be willing to consider them. 

 Professor Jennings, in his studies of Paramecium, has isolated a num- 

 ber of races or varieties which possess constant characters by which they 

 can be recognized, and which are believed to be common throughout 

 the country. 



1 University of Colorado Studies, IX., pp. 65-74; Science, September 9, 19J0. 



