P.HYSIOLOGY OF THE INVERTEBRATA. 391 



three. When the tripod which is moviug has come to the 

 developed as to give the insect a praying attitude — ^these 

 limbs are used as prehensile organs. The anterior limbs of 

 Gryllotcdpa (the mole cricket) are modified to suit this insect 

 to its burrowing habits. In those insects which swim, such 

 as Dytiscus, the tarsi are generally flattened, ciliated, and 

 disposed like oars. In fact, Dytiscus possesses organs of 

 natation, of burrowing, of reptation, and of flight. This Oole- 

 opterpus insect is, in a sense, comparable to the great epic 

 poet's fiend in the nature of its various movements, and 

 also the different elements in which it is capable of living : — 



" Through strait, rough, dense, or rare, 

 With head, hands, wings, or feet, pursues its way. 

 And swims, or sinks, or wades, or creeps, or flies." * 



These wonderful modifications of a general plan are cei*tainly 

 strong points in the theory of natural selection. "It may 

 metaphorically be said that natural selection is daily and 

 hourly scrutinising, throughout the world, the slightest 

 variations ; rejecting those that are bad, preserving and 

 adding up all that are good ; silently and insensibly working, 

 whenever and wherever opportunity ofiers, at the improvement 

 of each organic heitig in relation to its organic and inorganic 

 conditions of life. We see nothing of these slow changes in 

 progress, until the hand of time has marked the lapse of ages, 

 and then so imperfect is our view into long-past geological 

 ages, that we see only that the forms of life are now different 

 from what they formerly were." f 



Mr. H. H. Dixon, of Trinity College, Dublin, has recently 

 made some observations on the locomotion of various insects, 

 and he finds that in the case of those which move quickly, the 

 best method for observation is instantaneous photography. 

 Instantaneous photographs of moving flies show that they 

 move the front and hind legs of one side almost simultanpously 

 with the middle leg of the other, while they stand on the other 



* Milton's Paradise Lost. 



t Darwin's Origin of Specks (6th ed,), p. 65. 



