Evolution and Progress 189 



every year. The shelled Cephalopods, of which the pearly 

 nautilus is a living representative, existed in past ages in vast 

 numbers and in many different species. Today the whole 

 tribe is represented by very few species and very few in- 

 dividuals. The saber- tooth tiger was undoubtedly fit to live 

 when he did live. Yet he has gone and has left no descend- 

 ants. We can only guess what advantages the Irish elk found 

 in the tremendous development of his horns, or what ad- 

 vantage the stegosaur derived from the immense frill which 

 he carried on his back. 



It is not necessary, indeed, to assume that these mon- 

 strous structures had any adaptive value whatever. Both 

 the elk and the stegosaur died out in time, and there is rea- 

 son to believe that aside from any difficulties provided by 

 their enemies or competitors, the very development of these 

 specialized structures were themselves handicaps to success- 

 ful living. We are tempted to generalize from the fate of 

 these extinct inhabitants of the earth, because we find in so 

 many cases the extreme elaboration of ornamentation and 

 unessential structures. We are tempted to moralize on the 

 dangers of excessive attention to the ornate, the over-refine- 

 ment of detail. But we do not need to extend our specula- 

 tions for the present: the facts may be allowed to speak for 

 themselves. Modification of type has taken place in the 

 course of the generations. Some of the modifications have 

 found themselves incapable of maintaining a foothold, some 

 of the modifications have left descendants which in turn 

 have carried a tendency to the point of self-destruction. 



Parasitism represents a type of adaptation which again 

 carries with its many advantages the obvious danger of over- 

 specialization. The parasite that confines itself to a particu- 

 lar host may continue indefinitely to furnish its share of 

 the earth's inhabitants, but if anything should happen to 

 the host species, the parasite is doomed. When a parasitic 

 species is exterminated because for one reason or another the 

 host species failed to supply it with a suitable medium, we are 

 again disposed to moralize on the wickedness and the ultimate 



