Optimum Temperature 155 



range no regular correlation exists between temperature and time of 

 development. On the other hand, some life processes go forward 

 only within a very narrow thermal range. Everyone who has raised 

 chickens knows that it is impossible to persuade the eggs to hatch 

 more quickly by increasing the temperature. Although the hen's egg 

 has no temperature control of its own, it will hatch only if maintained 

 between 40°C and 41°C and only after 21 days of incubation. 



Optimum Temperature. The foregoing discussion of the influence 

 of temperature on the rates of biological processes leads to a con- 

 sideration of the optimum temperature. However, the concept of 

 the optimum in ecology is a slippery customer unless the life process 

 concerned is specified. The optimum temperature may be considered 

 that value at which a certain process goes on the fastest. But the tem- 

 perature for the maximum rate often varies considerably for different 

 processes within the same organism and also for the same process at 

 different stages in the life cycle. 



Among plants in the temperate zone the optimum temperature for 

 germination usually differs markedly from the optimum value for the 

 fruiting process, and the optimum heat condition for photosynthesis 

 may occur at a value different from either of the other two. Similar 

 variations in thermal relations are found among animals. The optima 

 for the various developmental stages are frequently found at values 

 widely different from the optimum for the adult. The eggs and larvae 

 of terrestrial species usually require a higher temperature than the 

 adult stage, but notable exceptions are provided by many animals as, 

 for example, by the corn borer, and the eastern brook trout. The trout 

 spawns in October or November, and the eggs can develop at 4°C or 

 less. The optimum temperature for the development of trout eggs is 

 8°C. The adult trout, on the other hand, do not feed much and there- 

 fore do not grow until the water is warmer than 10°C. The optimum 

 for the growth of the adult falls between 13°C and 16°C. These ex- 

 amples are sufficient to indicate that no one point on the thermal scale 

 can be designated as the optiinum for the entire growth of an organism, 

 but that optimal values differ according to the life stage of each 

 species. 



The most favorable temperature for survival often differs consider- 

 ably from the optimum value for growth, reproduction, and other 

 life processes. Mackerel eggs develop at the fastest rate at 21°C, but 

 in the same investigation the survival of the eggs was found to be best 

 at 15°C. Other experiments showed that the life span of the common 

 water flea, DapJmia magna, is greatest at 8°C (Table 12). In con- 

 trast, the shortest time for the production of the first batch of young 



