EVOLUTION AND NATURAL SELECTION 23 



secretes a plasmatic fluid quite inviting to the female. This 

 pale green insect, in the male sex, has his forewings also so 

 modified that when he shuflBes them together over his body they 

 produce a high-pitched shrilhng, which attracts the female, 

 if within hearing distance. Then he brings into action both 

 the wings and the organ above referred to. The shrilhng 

 sound, together with the secretion in the hollow glands on his 

 back, entices her to cKmb on him, and there she partakes of 

 his loving cup. This performance is preliminary to the nuptial 

 joining of the sexes. 



The instrumental sounds made by the Orthoptera (grass- 

 hoppers, locusts, and crickets) are produced by various mechan- 

 isms, the basis of which is usually in the nature of file-like 

 surfaces rubbed together. The music or sounds thus produced 

 make up the majority of the medley of sounds to be heard 

 in late Summer while walking through the meadows. These 

 stridulating organs are secondary sexual in character, and are 

 possessed only by the male. They, as Weismann says, "are 

 of undoubted importance to the sex. These can certainly be 

 attributed with great probabihty to sexual selection." '■ 



SXJMMARY OF NATXmAL SELECTION 



One of the most familiar criticisms of natural selection, 

 says Brooks, is "that it does not produce, but only preserves 

 the fitness which exists. It does not show why there should 

 be any fit to survive, but only why the unfit are exterminated. 

 They who challenge the sufficiency of natural selection on these 

 grounds must remember that all science is inadequate in the 

 same degree; for in no case does science tell us why natural 

 phenomena do occur in order, although it does tell what order we 

 may reasonably expect. However, the common verdict of man- 

 Idnd is that scientific knowledge is very adequate and sufficient 

 for all the practical needs of hving beings, even if it does fail to 

 show us In nature any efficient cause for phenomena." 



Natural selection, says Jordan,^ "very hkely is not all- 

 machtig. Darwin never claimed that it was. But it is 

 potent for all that, and the other factors in evolution work with 



1 "Studies in Theory of Descent," P- 62. 



2 Science, N. S., Vol. XXX, 1909, p. 528. 



