310 PRINCIPLES OF ANIMAL BIOLOGY 



several species of Nautilus, sole survivors of a once flourishing family 

 (the tetrabranchiate cephalopods), now found only at places in the 

 Pacific and Indian Oceans. 



Size of Range. — Equally striking are the different sizes of ranges 

 occupied by the various forms. When groups of high and low taxonomic 

 rank are compared, as orders with genera, inequalities are to be expected. 

 One simple reason is that the higher groups are made up of a number 

 of lower ones. When those of the same rank occupy very unecjual areas, 

 an explanation is not always easy. Particularly important in the theory 

 of distribution are unequal ranges of species. Some ranges are very 

 small. One species of ant is found only in the Garden of the Gods in 

 Colorado, another species occupies much of North America. Kirtland's 

 Warbler, not including its migration routes, exists only as a few indi- 

 viduals in a very limited area, while the American Robin numbers 

 millions of individuals and covers a continent. Among plants, a species 

 of Oenothera includes only 500 to 1000 individuals and is known only 

 in a mountain range in southern New Mexico. One of the spadefoot 

 toads, already mentioned, occurs only in the islands off Florida and per- 

 haps at the extreme tip on the mainland, while another species of the 

 same genus has a range a thousand miles wide. 



When there appears to be no difference in the tolerance, rate of 

 reproduction, or means of locomotion of two species, a tempting expla- 

 nation is a difference in age. This is thought to be the reason for the 

 very unequal ranges of four species of tree frogs (genus Hyla). Hyla 

 versicolor is found from southern Canada to the Gulf states, and west to 

 a line between Montana and central Texas; H. squirella extends from 

 Virginia to Florida, west to Texas and Indiana; H. gratiosa from South 

 Carolina to Florida and Mississippi; and H. evittala only along the 

 Potomac and York Rivers in Virginia and New Jersey. The species 

 believed to be the younger have the smaller ranges, and the explanation 

 may be simply the shorter time they have had to spread. 



This idea has been developed as the "Age and Area" hypothesis, 

 and has been applied more to plants than to animals. In accord with 

 it is the fact that on the average groups of higher taxonomic rank (orders, 

 for example) occupy areas larger than those belonging to groups of 

 lower rank (genera, let us say). In general, the higher groups are older, 

 and have had longer time to disperse. Some paleontological support for 

 it is also claimed, for when the ages of taxonomic groups can be judged 

 from the geological periods which furnish their earliest known fossils, the 

 older ones again have the larger average ranges. 



There are known exceptions to the rule, however, and probably 

 many which are not known. Two species of shophord's-purse differ in 

 the number of chromosomes in their cells, one having just twice as many 



