CHAPTER XIII. 
THE ORIGIN 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 faune. 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. 
Reasoning @ 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 Paleozoic strata ; the laws of the succession of types ; the 
correlation existing between the development of the indi- 
