188 THE POPULAR SCIENCE MONTHLY. 



Each of the geological periods has its dominating representative 

 type of life. Perhaps it may he asked : " How can we be satisfied that 

 the members of this long series are strictly the successive descendants 

 by evolution from older forms, and in their turn the progenitors of the 

 later ? How do we know that they have not been introduced by sud- 

 den creations, and removed by sudden extinctions ? " Simply for this 

 reason : The new groups make their appearance while yet their pred- 

 ecessors are in full vigor. They come under an imperfect model 

 which very gradually improves. Evolution implies such lapses of 

 time. Creation is a sudden affair. 



A striking illustration of this is offered by two of the most im- 

 posing types of life, the reptile and the mammal. The former is the 

 characteristic of the Secondary, the latter of the Tertiary period. In 

 the Secondary, when reptile life was at its culmination, there were 

 reptiles flying in the air, swimming on or in the sea, crawling on the 

 land, or climbing the trees. After this type of life had reached its 

 culmination, and extinction began to set in, that process went for- 

 ward in a gradual and orderly way. The flying lizards were the first 

 to disappear, then those of the sea ; they now have scarcely any rep- 

 resentative left. The fluviatile and terrestrial ones, though greatly 

 diminished both in numbers and size, still maintain a struggle for 

 life; but the complete dying out of animated forms, though irre- 

 sistible, requires for its completion countless centuries. 



While reptile life was in full vigor, mammal life was introduced. 

 It came under the lowest forms, the imperfect orders appearing first. 



What does this coexistence of two different forms of life, through 

 immense lapses of time the one declining and on its way to disap- 

 pearance, the other marching forward to increase what does this 

 overlapping mean ? Not sudden creation, but slow development. 

 The environment is slowly becoming unsuitable to the one, and slow- 

 ly becoming suitable to the other. 



If time permitted, I would ask your close attention to rudimentary 

 organs, for they illustrate strikingly the theory of evolution. They 

 are organs existing in an apparently undeveloped and useless condi- 

 tion, such, for instance, as the incisor teeth in the midbone of the 

 upper jaw in embryos of common cattle, the rudimentary wings of the 

 penguin and dodo, the mamma? of the male mammalian, the subcuta- 

 neous feet of certain snakes. In the embryos of whales teeth are found 

 in the jaw, precisely as we find them at birth in the human infant. In 

 the latter instance, we think we see a wise provision and foresight of 

 Nature, which does not give to man these masticatory organs before 

 the time they are wanted. But what are we to make of the parallel 

 case of the whale? Shut up as these rudimentary teeth are in the in- 

 terior of the jaw, never to be developed and never to be used, does 

 not that look something like a useless work? And why has Nature, 



