214 COMPARATIVE ZOOLOGY 
feet; and those with long beaks, large feet. The long 
limbs of the Hound are associated with a long head. <A 
white spot in the forehead of a Horse generally goes with 
white feet. Hairless Dogs are deficient in teeth. Long 
wings usually accompany long tail- feathers. White Cats 
with blue eyes are deaf. A Sheep with numerous horns 
is likely to have long, coarse wool. Homologous parts 
tend to vary in the same manner; if one is diseased, an- 
other is more likely to sympathize with it than one not 
homologous. This association of parts is called correda- 
tion of growth. 
6. Relations of Number, Size, Form, and Rank. 
The Animal Kingdom has been likened to a pyramid, 
the species diminishing in number as they ascend in the 
scale of complexity. This is not strictly true. The num- 
ber of living species known is about 400,000, of which 
nearly nineteen-twentieths are Invertebrates. The Ani- 
malcules (not reckoned in this count) are innumerable. 
But next, the Articulates are the most numerous, then 
Vertebrates, Mollusks, and Radiates. Of Vertebrates, 
Fishes are most abundant; then follow Birds, Mammals, 
and Reptiles. 
The largest species usually belong to the highest classes. 
The aquatic members of a group are generally larger than 
the terrestrial, and the marine than the fresh-water. The 
extremes of size are an Infusorium, 3,55 of an inch in di- 
ameter, the smallest animal ever measured, and the Whale, 
one hundred feet long, the largest animal ever created. 
The female is frequently larger than the male, as of the 
Nautilus, Spider, and Eagle. The higher the class, the 
more uniform the size. Of all animals, Insects and Birds 
are the most constant in their dimensions. 
Every organism has its own special law of growth: a 
Fish and an Oyster, though born in the same locality, de- 
4 
