48 DARWINISM TO-DAY. 



be explicable by selection, because we are accustomed to 

 postulate thoroughly fortuitous and all-inclusive variation; 

 but that the tooth standing next to it shall have varied al- 

 ways in exactly the same way so that the result of its de- 

 velopment shall make it identical with the other one, is 

 inexplicable by selection on a basis of fortuitous variation, 

 but rather indicates that the change of form is ruled by law 

 which we do not know. The attempt to discover it is the 

 most imperative task for biologists to undertake." 



Wolff 28 follows this argument farther by discussing other 



particular examples, but they are all of the type of the one 



Spencer's pic- J ust set out - Spencer pictures the situation of 



tnre of the inu- the herbivorous animals in a country of in- 



tility of advan- 



tage in a single clement climate and populated by numerous 

 carnivores. Now those herbivores which have 

 the finest hearing will be soonest aware of the approach of 

 the tiger, but those with keenest sight or most perfect sense 

 .of smell will also perceive, as soon, that it is time to flee. But 

 what advantage over others will the first start in flight give 

 them? Others less delicately endowed with sense organs 

 but swifter of foot will, although starting a little later, have 

 as good a chance to escape because of their more rapid run- 

 ning. Later may come snow and terrible cold. Those in- 

 dividuals best endowed with sense-organs or swiftest of foot 

 will not necessarily be the most enduring or the best 

 equipped with instincts to find shelter. The climate may 

 decimate those which selection on the basis of special senses 

 or speed has saved. But after the cold may come the sum- 

 mer drought. Those most heavily furred or warmest- 

 blooded which have successfully endured the low temperature 

 and snow and ice of winter should be the first to suffer from 

 the attacks of sun and drought and lack of food in the 

 summer. Thus no individual has, because of advantage in 

 any one character, any real and complete superiority which 

 guarantees it success in all the phases of the struggle for 



