120 THE GROWTH OF GROUPS 



small, a few square miles. Sports, when met with, were 

 described. Having observed and described the present 

 state of the group, he next endeavoured to ascertain the 

 steps by which the group came to be in that state. 

 Although this seems to be the only way of reading the 

 past history of a group, i.e. of ascertaining the method 

 of evolution, few persons have as yet followed it. 



Let us now see what Tower found. He examined 

 207,891 specimens of the commonest species, Leptinotarsa 

 dece.mlineata, which were taken during nine years from 

 seven widely separate parts of the United States. Among 

 them he found 118 sports of nine different kinds. Some 

 kinds were more common than others. Thus the kind 

 pallida was found on sixty-three occasions, the kind 

 melanicum thirty-one times, the other seven kinds were 

 found only once or twice ; a striking form called tortuosa 

 was found three times. Some of these sports are shown 

 on Plate II. 



The idea of species is not confined to naturalists, every 

 one has his own idea as to what constitutes a kind of 

 animal. Nearly every one will agree that the beetles 

 shown on Plate II. are of different kinds. If a person 

 be questioned as to what he understands by a kind of 

 animal, he will perhaps select an animal and state his 

 belief that the ancestors of that animal always were, 

 and its descendants always will be, like it. But the evi- 

 dence is perfectly clear to the contrary, one kind may give 

 birth to another occasionally, and the new kind may 

 henceforth produce descendants like itself. It will be long 

 before it is widely known that an animal of one kind may 

 give birth to an animal of another, that is to say of a 

 kind slightly but obviously different. Eventually it must 



