144 . GENERAL EVOLUTION. 



acceleration ? remains unanswered. One can not understand why 

 more liigbly oxidized blood should hasten the growth of partition 

 of the ventricle of the heart in the serpent, the more perfectly to 

 separate the aerated from the impure fluid ; nor can we see why a 

 more perfectly constructed circulatory system, sending purer 

 blood to the brain, should direct accelerated growth to the cere- 

 bellum or cerebral hemispheres in the crocodile'. 



b. In Characters of the Specific Kind. — Some of the charac- 

 ters usually placed in the specific category have been shown to be 

 the same in kind as those of higher categories. The majority are, 

 however, of a different kind, and have been discussed several 

 pages back. 



The cause of the origin of these characters is shrouded in as 

 much mystery as that of those which have occupied the pages im- 

 mediately preceding. As in that case, we have to assume, as 

 Darwin has done, a tendency in Nature to their production. 

 This is what he terms "the principle of variation." Against an 

 unlimited variation the great law of heredity or atavism has ever 

 been opposed, as a conservator and multiplier of type. This 

 principle is exemplified in the fact that like produces like — that 

 children are like their parents, frequently even in minutiae. It 

 may be compared to habit in metaphysical 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, only a proj^ortion 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 proposition which distin- 

 guishes his theory from other development hypotheses, and which 

 is stated in brief by the expression, "the survival of the fit- 

 test." Its meaning 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 supply, or 

 with rivals in its pursuit, dwindle and are sooner or later ex- 

 tirpated ; while those which are adapted to their surroundings, 

 and favored in the struggle for means of life and increase, pre- 

 dominate, and ultimately become the centers of new variation. 

 "I am convinced," says Darwin, "that natural selection has 

 been the main, but not exclusive, means of modification." 



