J6 



LIIK : OUTLINES OF CiENKKAL BIOLOGY 



"forms" or varieties are described by Jordan in one of the com- 

 monest of small Crucifcrs, the whitlow-grass or Draba vcrna ; and these 

 are no longer fluctuating but breeding true. Again, Lotsy speaks 

 of the bewildering diversity exhibited by a series of about two 

 hundred s|)ecimens of the Common Buzzard [Buteo buteo!) in the 

 I-eyden Museum, "hardly two of which are alike". It is difBcult 

 to see much difference between one reeve and another, but it is 

 as ditTicult to find two ruffs that look alike! Whenever one settles 

 down to work at sjx'cies. one is confronted with the difficulty that 

 so many of them are in llux. Vet others again seem to have settled 

 down in unchanging stability. 



The facts and factors of organic evolution will be discussed in 



Fig. «). 



Ihrtc Miilations of the I'rond in the Hart's Toiiriu' I-Vrn {Scolo[>endrium 

 vulf^atc). I. Tlic normal tyi>c. After l^)\ve. 



their prf)iHT place, here we arc only concerned with pointing out 

 that variability and with it evolvability— must be ranked as one 

 of the fiindamental characteristics of living beings. 



Whatever theory wc hold as to the factors of organic evolution, 

 we must leave rof)m for the bent bow of endeavour. The organism 

 selects stimuli from its environment and often moves from one 

 environment to another; the organism is often experimental, 

 moulding itself by its efforts; the organism tests the newnesses of 

 its inheritance in its ceaseless trafficking with circumstances. The 

 organism is what some have called a "historic being", meaning 

 that it comes into existence with an inheritance of organisation 

 and impulses a rule of life, what from Plato's time has been 



