retreating_ is not a sentence, nor is _worms kept in pots_, in 3.
_Retreating_ and _kept_ here express action, but they are not predicates;
they do not assert. You learned in Lesson 16 that certain forms of the verb
do not assert. _Of the worm's body_ modifies what? _Then_ and _very often_
do what?

If you will compare these numbered sentences with Mr. Darwin's, you will
see how two or more sentences are put together to make one longer sentence.
You see Mr. Darwin puts our sentence 1 after _says_ to tell what Morren
says. What word here helps to bring two sentences together? Change this
sentence about so as to make _says Morren_ come last. See how many other
changes you can make in the arrangement of the words and groups of words in
this sentence. What two words are used to join 3 and 4 together? Notice
that these sentences are not joined so closely as 1 and 2, as is shown by
the semi-colon. Notice that _if_ has much to do in joining 5 and 6. These
are more closely joined than 3 and 4, but not so closely as 1 and 2. How is
this shown by the punctuation? Put 5 and 6 together and change their order.
Find, if you can, still another arrangement.

+To the Teacher+.--It is very important that pupils should learn to see
words in groups and to note their offices. If difficulties and
technicalities be avoided, such exercises as we suggest above may be begun
very early. They will lead to an intelligent observation of language and
will prepare the way for the more formal lessons of the text-book.

If time can be had, such exercises may profitably be continued through the
second and third paragraphs of the selection above.

We have said elsewhere that the sentence exercises on this selection from
Darwin may follow Lesson 30, but the teacher must determine.