404 ENTOMOLOGY 



blue spruce association than anywhere else. Hymenoptera. The social 

 wasp, Polistes variatus, nests from the valley of the Rio Grande up into 

 the spruce forest at eight thousand feet. The pigeon Tremex, T. columba, 

 is equally abundant in the cottonwoods of the valley and in the Douglas 

 spruce of the mountains. Hemiptera. Gerris remigis, the well known 

 water strider of eastern states, is found on all suitable ponds and streams 

 in both valley and mountains. The plant-feeding bug, Lygus pratensis, 

 is as ubiquitous here as it is elsewhere in the United States. 



VII. SUCCESSION 



" Succession is no doubt one of the most important and widespread 

 of the phenomena discovered by the ecologists up to the present time. 

 Simply stated, it means that on a given fixed area organisms succeed one 

 another, because of changes in conditions. These changes make 

 impossible the continued existence of the forms present at any given 

 time; with the death or migration of such forms, others adapted to the 

 changed conditions occupy the area, whenever such adapted forms are 

 available. The changes referred to result from physical or biological 

 causes, or combinations of the two. It is probable that the causes of 

 the changes are frequently complex combinations of various factors. 



" We have among the physical causes changes in climate and changes 

 in topography. All degradation of land is a cause of succession. Such 

 geological processes are well understood and treated in textbooks on 

 geology and physiography. 



"The biological causes of succession lie chiefly in the fact that organ- 

 isms frequently so affect their environment that neither they themselves 

 nor their offspring can continue to live at the point where they are now 

 living. Every organism adds certain poisonous substances to its 

 surroundings, and takes away certain substances needed by itself. It 

 frequently thus so changes conditions that its offspring cannot live and 

 grow to maturity in the same locality as the parents. However, by 

 these same processes it prepares the way for other organisms which can 

 live and grow in the conditions thus produced." (Shelford.) 



"The general growth or evolution of environmental conditions 

 and the communities which belong to them are included under succes- 

 sion. The word succession is used in three distinct senses. We speak 

 of (a) geological succession, (b) seasonal succession, and (c) ecological 

 succession." (Shelford.) 



Geological. "Geological succession is primarily a succession of 

 species throughout a period or periods of geological time. It is due 



