152 EVOLUTION OF LIFE. 
reptiles of the Tartar and Arabian deserts, the Great Sahara, 
and the sands of Arizona and California. There is also a 
tendency to produce spiny forms in such places; witness 
the Stellios and Uromastix and Cerastes of the Sahara, the 
Phrysonomas and Horned Rattlesnake of Southwestern 
America. The vegetation of every order, we are also in- 
formed, is in these situations extremely liable to produce 
spines and thorns.” 
Every one is aware of the great difference in size and 
color exhibited by the male and female of birds, butterflies, 
etc., of male animals being armed with weapons, like the 
horns of deer, the cock's comb, etc. Mr. Darwin supposes 
these organs to have arisen through what he calls Sexual 
Selection. Thus, at breeding-time the number of male 
deer exceeds that of the حر‎ hence there is invariably a 
fight, and the deer with the سر‎ horns gets the better of 
his rivals: naturally their posterity will be characterized 
by large horns. This process, continued t! hrough genera- 
tions, finally results in the production of the antlers of the 
male deer. But, as Mr. Herbert Spencer observes, large 
horns require large muscles to move the head, large mus- 
cles must be supplied with sufficient nutriment, which is 
brought to them by large arteries, which necessitates a 
powerful heart, and so on indefinitely. The voice of the 
singing birds is supposed to have arisen in the same way, 
for of the. male birds those who sing best are chosen by 
the females for their mates. The voice is therefore con- 
tinually improved from generation to generation. The 
male Crickets, Grasshoppers, Katydids are equally remark- 
able for the noise they can make. The incessant “ Katydid 
she didn't” is produced by one wing being played on by the 
other wing, like a fiddle and bow. “All observers agree 
that the sounds serve either to call or excite the mute 
and Mr. Darwin quotes Mr. Bates as stating that 
the male of the European field-cricket “has been observed 
7 
' females ; 
