I >7 ) 171 



in minutiae. It may be compared to habit in metaphys- 

 ical matters, or to that singular love of time or rhythm 

 seen in man and lower animals, in both of which the 

 tendency is to repeat in continual cycles a motion or 

 state of the mind or sense. 



Further, but a proportion of the lines of variation is 

 supposed to have been perpetuated, and the extinction 

 of intermediate forms, as already stated, has left isolated 

 groups or species. 



The effective cause of these extinctions is stated by 

 Darwin to have been a "natural selection" a proposi- 

 tion which distinguishes his theory from other develop- 

 ment hypotheses, and which is stated in brief by the 

 expression, "the preservation of the fittest." Its mean- 

 ing is this : that those characters appearing as results 

 of this spontaneous variation which are little adapted to 

 the conflict for subsistence, with the nature of the sup- 

 ply, or with rivals in its pursuit, dwindle and are sooner 

 or later extirpated ; while those which are adapted to 

 their surroundings, and favored in the struggle for means 

 of life and increase, predominate, and ultimately be- 

 come the centers of new variation. " I am convinced," 

 says Darwin, " that natural selection has been the main, 

 but not exclusive, means of modification." 



That it has been to a large extent the means of pres- 

 ervation of those structures known as specific, must, I 

 think, be admitted. They are related to their peculiar 

 surroundings very closely, and are therefore more likely 

 to exist under their influence, Thus, if a given genus 

 extends its range over a continent, it is usually found to 

 be represented by peculiar species one in a maritime 

 division, another in the desert, others in the forest, in 

 the swamp or the elevated areas of the region. The 



