He who will carefully examine the flowers of orchids for himself will not deny the existence of the above series of gradations--from a mass of pollen-grains merely tied together by threads, with the stigma differing but little from that of the ordinary flowers, to a highly complex pollinium, admirably adapted for transportal by insects; nor will he deny that all the gradations in the several species are admirably adapted in relation to the general structure of each flower for its fertilisation by different insects.

In this, and in almost every other case, the enquiry may be pushed further backwards; and it may be asked how did the stigma of an ordinary flower become viscid, but as we do not know the full history of any one group of beings, it is as useless to ask, as it is hopeless to attempt answering, such questions.
We will now turn to climbing plants.

These can be arranged in a long series, from those which simply twine round a support, to those which I have called leaf-climbers, and to those provided with tendrils.

In these two latter classes the stems have generally, but not always, lost the power of twining, though they retain the power of revolving, which the tendrils likewise possess.

The gradations from leaf-climbers to tendril bearers are wonderfully close, and certain plants may be differently placed in either class.