What does Shakespeare mean in Sonnet 12 by: "And nothing 'gainst Time's scythe can make defense / Save breed, to brave him when he takes him hence"?

In the intricately wrought Sonnet 12, Shakespeare weaves
an underlying comparison, through metaphor, between the
natural elements of time and the aging of humanity and decay of human beauty. A
second metaphor compares annual time to a human life span.
The sonnet is a lament of sorts that grows from the poetic speaker's contemplation of
the signs of the passage of annual time and the juxtaposition of those signs with
thoughts of the beloved beauty being addressed:

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When I ... / see the brave day sunk in hideous
night; / ... /
And sable curls all silver'd o'er with white; / ... /

Then of thy beauty do I question make,
That thou among the wastes
of time must go,

The speaker
carries the contemplation further, first, by saying that "Since sweets and beauties" of
humanity fade and die even while a younger generation grows
up,

Since
sweets and beauties do themselves forsake
And die as fast as they see others
grow;

and, second, by
arriving at a conclusion. It is the conclusion to the speaker's contemplations that is
represented in the lines that form the ending couplet of the Shakespearean (or English)
sonnet:

And
nothing 'gainst Time's scythe can make defence
Save breed, to brave him when
he takes thee hence.

Recall
that the underlying metaphor of the sonnet is the
comparison of natural elements of time's passage to the life, beauty, old age and death
of humanity, or, more specifically, a beloved human:

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When I behold the violet [flower] past [its]
prime,
And [your] sable [colored] curls all silver'd o'er with white; / ...
/
And summer's green [fields] all girded up in [harvest]
sheaves
Borne on the [funeral] bier with white and bristly beard [of old age],
….

The
underlying metaphor is continued in the couplet wherein a
natural sign of the passage of annual time again precedes a thought related to the life
and death of the beloved. First, natural annual time passing is called up through the
imagery of autumn's harvest when a scythe is used to harvest grain from the field; when
it is harvest time, the year is almost over, and, according to the second metaphor, life
is almost over.

The image is then made an additional,
separate metaphor for universal time by giving time a persona and a possession: "Time's
scythe." Time is said to be unstoppable through the statement that there is "no defense"
against time's harvest, which is ultimate death: "And nothing 'gainst Time's scythe can
make defence."

The last line of the couplet is the
speaker's solution to the problem of the unstoppable march of time toward death: "That
thou among the wastes of time must go." Usually, Shakespeare's solution to the decay of
beauty and life is the immortalization of the beloved in poetry, and it might be said
that he was quite successful in his efforts! In the final line of this sonnet, a
different solution is offered, though it is a solution foreshadowed in the last line of
the third quatrain: "And die as fast as they see others
grow."

The speaker advises the beloved beauty to "breed,"
to bear children, so that when the scythe of Time comes, the beloved may put up a
courageous front and face Time stalwartly ["brave"] as death approaches: "to brave him
[Time] when he takes thee hence" into death. As a final note, the word "save" is used in
its meaning that is synonymous with "except"; so "Save breed" means "except
breed."