22 NATURE SKETCHES IN TEMPERATE AMERICA 



insects, supplying the plant with nitrogenous food. Thus the 

 insect and plant oscillations go hand in hand. Not only are 

 these adjustments of structure to the environment presumed 

 to be perfected by natural selection, but the physiological 

 adjustments of plants and animals to the climate, tempera- 

 ture, and humidity are so poised that without adaptation hfe 

 seemingly could not exist. 



In our foregoing account we have explained but a small 

 part of the great factors of evolution, treating natural selec- 

 tion in a brief resume. We will now outline a phase of 

 evolution which has proved a factor hand in hand with natural 

 selection, namely: sexual selection. 



Sexual Selection 



Sexual selection must necessarily operate in animals where 

 there are two sexes, such as birds, which exhibit quite a difference 

 in the ornamentation of their plumage. This difference between 

 the male and female coloration was thought by Darwin to have 

 been brought about by a rivalry between the males in a struggle 

 to win admiration from the opposite sex. This form of color 

 modification may go hand in hand with structure, such as the 

 wattles and combs and spurs which essentially belong to the 

 male. These are presumed to have been perfected in the battle 

 among themselves, the succes.sful combatant winning the female 

 and thereby transmitting these characters to the offspring. 



It is not always possible, in the nature of the case, to deter- 

 mine how far .sexual .selection of this sort plays a part in the 

 perfecting of these peculiar structures of the male, for after 

 all it may be that natural .selection has had something to do 

 also in perfecting them. On this very j)oint there was a differ- 

 ence of opinion between Darwin and his contemporary, Wallace. 

 The latter did not make the fine distinction of supposing sexual 

 selection to be a contributing factor in these cases, but relied 

 on natural .selection as being sufficient for their production. 



Even in insects we find the so-called secondary sexual char- 

 acters, but they are present only in the male. A most remark- 

 able example of this type of structure in insects occurs in the 

 striped meadow cricket, described at some length further on. 

 It has an alluring gland on the back between the wings which 



