BRIEF ESSAYS 195 



our tree of life. All the great branches are repre- 

 sented, all the important classes of animals have 

 made their appearance, even the vertebrates being 

 represented in the upper Silurian by fishes. Of 

 this tree the sub-kingdoms represent the great 

 branches, the classes represent their division, the 

 orders theirs, the family theirs, and so up to 

 species which represents the terminal twigs. The 

 abundance of specialized forms in the Silurian age, 

 that is, the many smaller branches that appear, and 

 the absence of two generalized forms, or main 

 branches, that must have preceded them, is one of 

 the main obstacles in the way of the evolution the- 

 ory, a theory of generic descent; but those parent 

 branches, as I have said, are hidden, the record of 

 them has not been found, probably never can be 

 found. 



It is very certain, not only from direct evidence, 

 but in the light of analogy, that the forces of nature, 

 vital and other, were much more active in the early 

 geologic ages than they are now. It was the youth 

 of the world ; why should they not be more active ? 

 Why should there not have been more fluids and 

 gases and more rapid growths and changes ? There 

 was more heat, doubtless more rapid evaporation, 

 and more copious precipitation. Our rivers and 

 lakes and watercourses are but a fraction of what 

 they were in comparatively recent geologic times. 

 This tree of life grew rapidly in those warm, moist 

 May and June days of the Silurian and Devonian 

 epochs. New species appeared with comparative 



