NEED TO CLASSIFY—BATTEN 511 
used to express current ideas regarding the relationship of organisms 
one with another. 
From the first, mankind classified the things he observed by a 
method which declared that “like things belong together.” This 
method was implicit in the first classification of living things and 
remains the chief method of classifying today. But it is a method 
that must be used with discretion for—as we shall see—one can very 
easily classify objects on the basis of their superficial resemblances, 
while overlooking important basic characteristics which may be some- 
what less obvious. 
As an example, we might say, “I will construct a category for 
animals that fly.” Such a single category would include many flying 
animals that were more or less related. But birds and bats would 
occupy the same category, because both possess flying appendages. 
Upon closer examination, however, we would note that the wings of 
a bird and a bat are actually quite different. Further examination 
of the other organs of birds and bats would show that, while these 
two animals are superficially alike, in detail they are not at all closely 
related. If we were sensible, we would change our classification to 
recognize these differences. 
Refinements of a classification—although considerably more subtle 
than in this example—are a daily and important part of work today 
that aims at achieving a framework which reflects the relative degree 
of relationship both between contemporary organisms and between 
the animal forms of the evolutionary past. 
Let me now relate a case that demonstrates how our knowledge 
has increased over the years and show some of the effects that this 
increased knowledge has had on classification. We will take the 
phylum Mollusca, and, within this phylum, chiefly the snails (which 
in classification are called the class Gastropoda). In addition to the 
snails, other classes belonging to this same phylum include: 
Phylum MOLLUSCA : 
Class PoLyPLAcoPHoRA (chitons) 
Class PELECYPODA (clams) 
Class CEPHALOPODA (octopus—chambered nautilus) 
Class ScapHopopa (tusk shells) 
We know that among the myriad of snails that make up the class 
Gastropoda (some 50,000 species are known to exist) there are sev- 
eral groups, collectively known as the “limpets,” which are peculiarly 
adapted to a rocky environment where surf or swift currents present 
rather rigorous conditions for life. These limpets have cap-shaped 
shells, and possess powerful muscles that enable them to adhere to 
rocks, even under the stress of pounding surf (figs. 2 and 3). 
