GENERAL ZOOLOGY 



those above and below the optimum. This illustrates the general principle 

 that the water relations of a species may not be the same at different phases 

 of the life cycle; humidities suitable for embryonic development may be too 

 low or too high for larval survival. Projecting these experimental results 

 into conditions in nature, we may conclude that a species of insect will occur 

 only in environments where the humidity is within the limits of the range 

 permitting development and survival of all stages, including the most sus- 

 ceptible, in its life cycle. 



It should be borne in mind that, as pointed out earlier, there is a direct 

 relationship between atmospheric humidity and temperature. Changes in 

 temperature affect the drying power of air, as markedly as thev determine the 

 solubility of oxygen in water. In comparing the results of humidity experi- 

 ments at different temperatures, it is often difficult to separate the direct 

 effects of temperature from those of temperature acting through humidity. 



General Considerations. We might analyze the limiting effects of a 

 great many additional physical factors, such as acidity or alkalinity of the 

 medium, light and other radiations, movements or currents in the medium, 

 and so on. But the examples just discussed should suffice to demonstrate the 

 general effectiveness of physical environmental factors. We may then proceed 

 to draw several broad conclusions, with the understanding that they are 

 supported by a great body of ecological data. 



^- For each species of animal, every physical factor in the environment 

 imposes its own specific and peculiar limits to survival and distribution, and 

 these limits may differ for different stages in the life cycle of the individual 

 animal. 



^- A particular species can exist only in environments in which none of its 

 tolerable limits of survival is exceeded, including those of its most susceptible 

 stages. 



^^ For any specific environmental factor, the limits of a given species may be 

 close together (narrow range of tolerance), or they may be widely separated 

 (broad tolerance). 



►- It follows that a species will be most stringently restricted in its distribu- 

 tion and survival by the single factor for which it has the narrowest range 

 of tolerance; this is the factor of most significance in determining the 

 existence of the species in any potential habitat. 



►- A species with no very narrow tolerances will be found almost universally 

 distributed in the general type of habitat to which it is adapted, but even a 

 related species with one or a few specific narrow tolerances will be less 

 broadly distributed. 



Animals are seldom found in habitats where one or more physical factors 

 approach the limits for survival of the species. Exceptional are animals, 

 incapable of migration, making a last stand for survival in an environment 

 undergoing relatively rapid changes in a direction unfavorable to the species. 

 More commonly, animals occur under conditions which approach the opti- 



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