8 THE HYPOTHESIS AND THEORY OF 



blossoms in their turn will lose their petals and 

 develop into fruit, all this must remain hidden 

 from the fly during its few hours of life. It might 

 therefore be tempted to believe that the blossoms 

 all around were created by the good God exactly 

 as it sees them, and will remain unchanged for 

 ever. The fly would be greatly mistaken, and 

 even as an ephemeral fly, if it had intelligence, 

 it might perceive some slight signs that the splendour 

 of the blossoms was not unchanging. It might 

 see that, in the course of a few hours, some buds 

 had already opened more fully, some blossoms 

 had lost their petals either partially or wholly. 

 The opening buds are those rare traces of modi- 

 fication of species which we can still prove to have 

 taken place, although within comparatively narrow 

 limits. If we continue the simile, the falling 

 petals are the species in process of extinction, 

 and the fallen leaves are the extinct species known 

 to us only as fossils, which reveal to us the fate 

 of all organic species on earth ; they come and 

 go and give place to their successors, and though 

 the duration of their existence may be reckoned 

 in thousands or even millions of years as is that 

 of many kinds of the Brachiopod genus Lingula 

 yet for them, as for each one of us, there is a beginning 

 and an end. But let us now abandon the simile. 



Upon what evidence is the doctrine of evolution 

 as a scientific hypothesis and theory based ? 



We must distinguish two kinds of evidence, 



