146 The Classification of Insects 



the broad fore-limbs of earth-burrowing insects like 

 the Mole-cricket are the direct result of a digging- 

 habit persevered in through many generations, while 

 the wingless condition of many female and parasitic 

 insects is simply due to their ancestors having given 

 up flying. It is hardly necessary to point out how 

 differently the natural selection theory accounts for 

 such facts as these. According to its advocates no 

 amount of the most vigorous digging on the part 

 of a primitive cricket would avail to provide his 

 descendants with broad fore-legs ; these organs are 

 believed to have been slowly specialised through a 

 long series of generations of crickets, who were 

 adopting the burrowing habit, through the selection 

 in each generation of those individuals best adapted 

 for burrowing, that is, those who possessed the 

 broadest and strongest fore-legs, the broadening not 

 being an acquired but a congenital character. And 

 there is no doubt whatever that congenital characters 

 are transmissible. In the same way it would be 

 believed that the loss of wings in the insects men- 

 tioned above being, for some reason, actually beneficial 

 to the species, individuals with the smallest wings 

 were selected through numerous generations until 

 those organs were almost or entirely lost. 



The efficacy of natural selection in fixing specific 

 differences depends largely on the indefiniteness of 

 variation. If animals vary, so to speak, in all 

 directions, natural selection must play a most im- 

 portant part in eliminating the unfavourable and 

 preserving the favourable characters. But if varia- 

 tion takes place along definite and determined lines, 

 the action of natural selection must be confined to 

 exterminating the weakly and feeble, and preserving 

 the healthy and strong ; it can do nothing to fix 

 specific characters, since it can have no " choice of 



