IN TROD UCTION. 5 



arisen by ordinary generation. Finally, by the con- 

 tinued development of the nervous system, the 

 intellect makes its appearance, rises higher and 

 higher, until it reaches the grade which we find in 

 man. Evolution is here, then, a comprehensive 

 theory, covering the slow growth of the whole uni- 

 verse by the simple action and interaction of force, 

 according to natural law. But even in this most 

 comprehensive sense the theory does not attempt to 

 explain the origin of the universe. To start with, it 

 assumes the existence of force and of a homogene- 

 ous mass of matter. 



Between the two extremes may be found almost 

 every grade, for evolution, when defined, means 

 simply that the past has given rise to the present 

 by the simple process of development according to 

 natural law. But, although the word may thus 

 mean almost any thing, and does have different sig- 

 nificance with different thinkers, it has, nevertheless, 

 in scientific literature, a tolerably definite meaning. 

 Among scientists the term seldom covers the 

 amount of ground which is included in Spencer's 

 theory, nor does it ever stop at the other meaning- 

 less extreme. As ordinarily used in most scientific 

 books to-day, evolution, organic evolution, and the 

 theory of descent are practically synonymous terms, 

 and each of these is used to indicate the theory 

 that all species of animals and plants existing to-day 

 have been derived from others living in the past, 

 by direct descent, and that they will themselves 

 give rise in the future to other still different species. 

 It further implies that if the histories of all animals 



