THE ORIGIN OF INSECTS 35 



lower to higher grades effected, and by what means did 

 insects come to differ from one another ? The answer to 

 this question is believed to lie with a great underlying 

 law, or principle, known as natural selection. In order to 

 understand its working we must recognise the fact that 

 while an animal resembles its parents in the main, it 

 nevertheless differs from them in more or less noticeable 

 details. Thus an ordinary dun cow may give birth to a 

 white calf, or (in an extreme case) to a calf with six legs. 

 Such variations may be useful or not useful, transmissible 

 or not transmissible; while the transmissible variations, 

 whether useful or the reverse, tend to be handed down 

 from parent to offspring through successive generations. 

 Nevertheless, we do not find that animals are encumbered 

 with meaningless or harmful characteristics. In some 

 way the useless variations are stamped out and disappear. 

 How is this accomplished ? We must remember, in the 

 first place, that any one species of plant or animal, were 

 its multiplication unchecked, would soon cover the whole 

 earth. Professor Huxley computed that the progeny of a 

 single aphid would, in ten generations, "contain more 

 ponderable substance than five hundred millions of stout 

 men ; that is, more than the whole population of China." 

 Yet this kind of thing does not happen, because the vast 

 majority of plants and animals meet destruction in early 

 life ; so that while an insect may lay a hundred eggs, the 

 individuals of the species are not thereby increased a 

 hundredfold, but remain approximately unchanged from 

 year to year. The survival or destruction of the in- 

 dividual is due, at least in large measure, to the principle 

 of natural selection. Those individuals survive which 

 have inherited the most favourable variations, whether of 

 structure, instinct, or habit, while those which are less 

 favourably endowed perish. Thus we speak of the 



