"The (Cobra)" vs. "An (elephant)", articles with nouns denoting a class

Question

[ 1 ] tells on p.5 that "Singular nouns denoting a class" are preceded by the definite article "THE" (Example: "The Cobra is dangerous"), while on page 7 (Table 6. THE INDEFINITE ARTICLE) it tells that "Singular countable nouns denoting a class" are preceded by indefinite articles ("An elephant never forgets").

I could not get what is the difference between the two cases mentioned.
Why can't I write according to the same rules "A Cobra never forgets" and "The elephant is dangerous"?

[ 1 ]
PRACTICAL REALISATIONS OF ENGLISH DEFINITE AND INDEFINITE ARTICLES

p.5 (of 8)

=================================  
 4. THE DEFINITE ARTICLE  
----------------------------------   
 USES          EXAMPLES 
-----------------------------------
 Singular        The Cobra is dangerous    
 nouns 
 denoting 
 a class
=======================================

p.7 (of 8)

=================================  
 6. THE INDEFINITE ARTICLE  
----------------------------------   
 USES          EXAMPLES 
-----------------------------------
 Singular        An elephant never forgets countable     
 countable 
 nouns 
 denoting 
 a class
=======================================

Update:
Irene asks:

How about the following example:

Is the happiness of the majority more important than the rights of the individual?

According to "First Certificate Language Practice" by Michael Vince this is the only way to say it. But couldn't you also say:

Is the happiness of a majority more important than the rights of an individual?

As I see it majority and individual both represent a class and an example of a class.

Answer 1

You can use either. There is functionally no difference between the following sentences:

The elephant will flee when confronted by danger.

An elephant will flee when confronted by danger.

Moreover, you can also use the plural with no article to say the same thing:

Elephants will flee when confronted by danger.

(Sorry, my phone doesn't want to let me select blocks of text for some reason, so I'm unable to highlight these examples.)

Answer 2

[ 1 ] tells that "Singular Nouns denoting a class" are prepended by the definite article "THE" on p.5 (Example: "The Cobra is dangerous")

In this case, it is assumed that The Cobra is being singled out from other snakes or animals. By prepending the class noun 'cobra' with the article 'the', you are placing extra emphasis on that class compared to other classes. Another common phrase that uses singular nouns in this way is: "The Lion is the king of the jungle." Here, each of the three class nouns is being singled out as being distinct from other comparitive classes. Changing any of the 'the's' to 'a' would change the meaning of the sentence:

• "A lion" implies that "a mouse" could have also been the king.
• "A king" implies that the lion is only one of many kings in the jungle.
• "A jungle" implies that the lion is only king within one jungle, not all jungles.

...while on page 7 (Table 6. THE INDEFINITE ARTICLE) it tells that "Singular countable nouns denoting a class" are prepended by indefinite articles ("An elephant never forgets")

Here, there is much less emphasis on singling out the class 'elephants'. The whole class is considered as a general group rather than a specific group. Other examples of using 'a' or 'an' in this way are: "A watched pot never boils" and "everybody loves a clown." We give consideration to all pots and all clowns rather than singling out a specific pot or clown.

Answer 3

How about the following example:

Is the happiness of the majority more important than the rights of the individual?

According to First Certificate Language Practice by Michael Vince this is the only way to say it. But couldn't you also say:

Is the happiness of a majority more important than the rights of an individual?

As I see it majority and individual both represent a class and an example of a class.