build in its
entirety another organism like the one of which it once formed a
component element.

Similarly with animals. Cut off a lizard's tail, and straightway a new
tail grows in its place with surprising promptitude. Cut off a
lobster's claw, and in a very few weeks that lobster is walking about
airily on his native rocks, with two claws as usual. True, in these
cases the tail and the claw don't bud out in turn into a new lizard or
a new lobster. But that is a penalty the higher organisms have to pay
for their extreme complexity. They have lost that plasticity, that
freedom of growth, which characterizes the simpler and more primitive
forms of life; in their case the power of producing fresh organisms
entire from a single fragment, once diffused equally over the whole
body, is now confined to certain specialized cells which, in their
developed form, we know as seeds or eggs. Yet, even among animals, at a
low stage of development, this original power of reproducing the whole
from a single part remains inherent in the organism; for you may chop
up a fresh-water hydra into a hundred little bits, and every bit will
be capable of growing afresh into a complete hydra.

Now, desert plants would naturally retain this primitive tendency in a
very high degree; for they are specially organized to resist
drought--being the survivors of generations of drought-proof
ancestors--and, like the camel, they have often to struggle on through
long periods of time without a drop of water. Exactly the same thing
happens at home to many of our pretty little European stone-crops. I
have a rockery near my house overgrown with the little white sedum of
our gardens. The birds often peck off a tiny leaf or branch; it drops
on the dry soil, and remains there for days without giving a sign of
life. But its thick epidermis effectually saves it from withering; and
as soon as rain falls, wee white rootlets sprout out from the under
side of the fragment as it lies, and it grows before long into a fresh
small sedum plant. Thus, what seem like destructive agencies
themselves, are turned in the end by mere tenacity of life into a
secondary means of propagation.

That is why the prickly pear is so common in all countries where the
climate suits it, and where it has once managed to gain a foothold. The
more you cut it down, the thicker it springs; each murdered bit becomes
the parent in due time of a numerous offspring. Man, however, with his
usual i