CHAPTER XIII. 



THE OKIGIN OF SPECIES. 



THE extinction of species was in some cases gradual, in 

 others sudden, so in all probability as different assemblages 

 of life became slowly extinct new forms as slowly originated 

 from them by genetic descent and took their places. While 

 here and there certain species, under favorable circumstances, 

 suddenly appeared, if we could have been there to look on, 

 it would perhaps have been as difficult to have observed the 

 process as it is at the present day to observe the changes 

 going on in the relation of existing faunas. We know, 

 however, that changes are going on in the world of life about 

 us, that the balance of nature is being disturbed. 



The nature of the evidence tending to prove that species 

 have originated through the agency of physical and biologi- 

 cal laws is mainly circumstantial, there being comparatively 

 few facts in demonstration of the theory, the direct act of 

 transformation of one species into another under the eye of 

 scientific experts having never been observed. 



Eeasoning d priori, we assume that organisms, both 

 plant and animal, have been created by development from 

 pre-existent forms because it agrees with the general course 

 of nature. All the events in geology, as in physics and as- 

 tronomy, being due to the operation of natural laws, it is 

 reasonably supposed that the production of all the species 

 of plants and animals from original simple forms, like the 

 Monera or bacteria, have been the result of the action of 

 natural law. The study of the early forms of life found in 

 the Palaeozoic strata ; the laws of the succession of types ; the 

 correlation existing between the development of the indi- 



