REDUCTION OF VARIABILITY. 129 



whether, reversely, the anatomical peculiarity made the habit 

 possible. We see the giraffe with its Jong neck and front-legs, 

 nicely fitted for browsing upon the foUage of trees, but it re- 

 mains an open question whether by a natural selection of the 

 individuals which could reach highest, the long neck was grad- 

 ually evolved, or whether a group of very long-necked individ- 

 uals found it possible to reach the leaves of trees, and thus 

 was able to migrate into regions where short-necked animals 

 could not Uve. The latter possibiUty appears more reasonable. 

 The circumstance of having a long neck may well isolate a 

 group of animals, and thus indirectly open the door for further 

 differentiation. 



In some tame animals we have good evidence for the fact 

 that an anatomical peculiarity may be the cause for a habit. 

 Anyone who has ever tried to teach tricks to a dog knows, that 

 it is very easy to teach a Dachshound or an Aberdeen terrier to 

 sit up, whereas it is very difficult to teach the trick to long-leg- 

 ged dogs. As in the very short-legged dogs it is the tibia and 

 fibula which are shortened most, these animals can sit down 

 flat on the groimd and hold the body up straight. Long-legged 

 dogs must balance themselves on their feet. Many short-legged 

 dogs learn the trick by imitation, and it is not rare to see a 

 Dachshound sit up to look over an obstacle or to warm itself 

 before the fire. 



For Darwin, natural selection was the only cause for specific 

 stability If a species, by continual selection, were wrought up 

 to the point where it as perfectly adapted to its environment, 

 natmral selection would keep it there. Darwin did, not think 

 that ever a group of organisms could be really pure, really 

 stable, in such a way, that in new surroundings it would not be 

 able to change and adapt itself to them It appeared to him, 

 that only continual selection could make a species pure, and 

 only continuous selection would keep it pure At present we 

 know, that all closed groups of organisms, groups which are in 

 some way protected from -admixture, speedily become stable 

 automatically. The total geno-variabihty of every group tends 



