Identifying the features themselves is a prerequisite to establishing life's hierarchy, so we need to look more closely at what we mean by "features." Observable features of anatomy are termed characters. Unique bones or unusual morphologies would all be characters. On the other hand, "observable features" would not include what something does - or how it does it. So, for example, "bites hard" is not a character, but perhaps "big jaw muscles" would be.
Characters acquire their meaning not as a single feature on a particular organism but when their distribution among a selected group of organisms is considered. For example, modern birds are generally linked on the basis of having feathers. All living feathered animals are birds and all birds have feathers. Thus, not only is a penguin a bird but so are eagles, ostriches, and kiwis: they all have feathers. And if someone told us that something is a bird, we could confidently predict that it too has feathers.
Because characters are distributed hierarchically, their position in the hierarchy is obviously dependent upon the groups they are being used to identify. Consider again the simple example of mammalian fur. Because mammals uniquely have this type of fur, it follows that, if you wanted to tell a mammal from a non-mammal (that is, any other organism), you need
1. A number of organisms in the world are fur-covered besides mammals; for example, bees and some spiders (like tarantulas) have a fur-like covering. But the fur on mammals is unique: it is made of unique proteins, grows in a unique way, and thus is different from these other furry or hairy coatings. When we refer to "fur" in this chapter, it is the mammalian type of fur to which we refer.
Figure 3.5. A cladogram. The cat and dog are linked by the characters listed at the hatch mark (or bar), just below the node. The node itself defines the things to be united; commonly a name is attached to the node that designates the group. Here, such a name might be "mammalian carnivores."
only observe that the mammal is the one that has that type of fur. On the other hand, the character "possession of fur" is not useful for distinguishing a bear from a dog; both have mammalian fur. To distinguish a bear from a dog, you'd need some character other than fur.
These distinctions are extremely important in establishing the hierarchy, and for this reason, characters function in two ways: as diagnostic characters and as non-diagnostic characters. The word "diagnostic" here has the same meaning as in medicine. Just as a doctor diagnoses a malady by distinctive and unique properties, so a group of organisms is diagnosed by distinctive and unique characters.
The same character may be diagnostic in one group, but non-diagnostic in a smaller subset of that group (because it is now being applied at a different position in the hierarchy). We saw that fur allowed us to tell a mammal from a non-mammal, but it can't distinguish one mammal from another: it wouldn't tell a bear from a dog.
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