840 THE AMERICAN NATURALIST. (Vor. XXXIX. 
For instance in the early members of the family of saber- 
toothed tigers (Machzerodidz), the upper canine teeth are elon- 
gated and flattened, making in such forms as Dinictis and 
Hoplophoneus a most efficient weapon. However, the elongat- 
ing went on gradually until in a form like Smilodon the great 
tushes reach far below the lower jaw, and to open the mouth wide 
enough to take in food below the weapons was practically impos- 
sible, so that most of the food probably went in between the 

a b £ 
Fic. 1, — Series of saber-toothed tigers’ crania. a, Dinictis ; 4, Hoplophoneus ; ej Smilodon. 
canines. At this stage the family, world wide in its distribution, 
died out. 
Taking this as an illustration of a variation going beyond its 
utility, there seems to the writer to be but one adequate expla- 
nation, namely, that as a special feature develops it attains à 
momentum which tends to carry it beyond the point of greatest 
utility. In so doing it may become a hindrance even to the 
point of exterminating its possessor, or may merely attain a per- 
fection, not detrimental, but without other explanation. 
Let this be applied to various groups. In the Radiolarian 
shell enough symmetry to maintain the balance of the animal 
would be expected, but when a mathematical perfection is shown 
in the repetition of every spine and ray in marvelous complexity, 
something more than utility has been subserved. The same 
may be said of the flesh spicules of sponges. At first being 
merely a deposition of something spinous in the flesh, so as to be 
disagreeable enough to protect the sponge from being eaten by 
small animals, these spicules are later carried to the perfection of 
