How Darwinism Stands To-Day 373 



Darwin's fluctuating variations, but the comparison breaks down 

 inasmuch as the living creature may be, as it were, fixed in one 

 of its oscillations, so that the variant makes a fresh start. Greater 

 disturbances of the polyhedron may make it roll over on to a 

 new face altogether, where it comes to rest again, only showing 

 once more the minor fluctuations around its new centre. The 

 new position corresponds to what is now called a mutation. 

 Studies in inheritance have shown that these mutations have great 

 staying power; they reappear persistently and intact in a cer- 

 tain proportion of the descendants. They are not liable to be 

 swamped by intercrossing, as Darwin supposed. The curious 

 fact is that the hereditary entailment of the fluctuating varia- 

 tions, which Darwin almost took for granted, requires more 

 demonstration to-day than does the hereditary entailment of 

 mutations. 



3 



Variations and Modifications 



Under the influence of persistent exercise, such as dancing, 

 the muscles of the legs increase in size, and the tendency to 

 increase may spread in an interesting way to other parts of the 

 body. Long-continued exercise of white rats increases the weight 

 of the heart, kidneys, and liver, on an average about 20 per cent. 

 Water-snails reared in cramped surroundings grow up dwarfish. 

 Goldfishes kept in the dark for three years become totally blind. 

 If the wan pigmentless Proteus from the Dalmatian caves be 

 exposed to light it becomes black, and the eggs laid by indi- 

 viduals kept in the light develop into dark larva?. Prolonged 

 pressure on a particular part of the skin often produces a thick- 

 ening or callosity. The colours of birds' feathers are sometimes 

 affected by the food they eat, as is well known in the case of 

 canaries and parrots. The stomach of the herring-gull changes 

 its character according to the diet whether it be fish or grain. 



Now all these changes are technically called "modifications"; 



