436 THE AMERICAN NATURALIST [Vol. LVI 



great numbers seriously interferes with the propagation 

 of certain plants. The overstocking of a range with 

 cattle or the presence of a vast colony of prairie dogs 

 may actually extirpate certain grasses in those districts, 

 and hordes of some rodents will prevent reforestation 

 in spots because all tree seeds are eaten as fast as pro- 

 duced. Poisonous plants work great havoc among range 

 stock at times, and although the amount of such devasta- 

 tion among wild forms has seldom or never been investi- 

 gated, it is doubtless an appreciable factor. In some 

 regions, bacteria and disease, including the smaller para- 

 sites, play a most important role. The tse-tse fly in por- 

 tions of Africa has rendered it utterly impossible for 

 certain herbivorous mammals to be kept in the infested 

 districts; the Stegomyia mosquito that is instmmental 

 in the spread of yellow fever probably caused the Mayan 

 suiwivors of this dread disease to abandon the ancient 

 civilization of Yucatan, which was at one time so densely 

 populated, and many ailments, comparatively harmless 

 to white men, who have developed a degree of immunity 

 to them, are largely responsible for the decrease in the 

 numbers within recent years of the more savage peoples. 

 From time to time either totally new bacterial diseases 

 appear or else old ones suddenly acquire new virulence, 

 and throughout the ages, such have undoubtedly killed 

 otf certain species from faunal divisions; and it is not 

 at all improbable that during the course of bacterial evo- 

 lution whole genera, or even families, have been extermi- 

 nated by this agency. 



It seems advisable to append to the present paper a 

 chart, or key, to the factors chiefly responsible for the 

 distribution and restriction of the ranges of living forms, 

 but this is submitted with considerable hesitancy. Most 

 of the factors mentioned are so interdependent upon 

 others that it is merely a matter of personal opinion as 

 to which heading they should be placed under. For in- 

 stance, it is impossible to decide whether the effect of a 

 cold mountain stream should better be listed under zonal 



