An example simile blending into metaphor in science writing,
"This brown object I will call "the boat-formed disc." It forms the middle portion of the posterior surface of the rostellum, and consists of a narrow strip of the exterior membrane in a modified condition. Its summit…is pointed, its lower end rounded, and it is slightly bowed, so as altogether to resemble a boat or canoe. It is rather more than 4/100 of an inch in length, and less than 1/100 in breadth. It is nearly rigid, and appears fibrous, but is really formed of elongated and thickened cells, partially confluent.
This boat, standing vertically up on its stern, is filled with thick, milky, extremely adhesive fluid, which, when exposed to the air, rapidly turns brown, and in about one minute sets quite hard. An object is well glued to the boat in four or five seconds, and when the cement is dry the attachment is wonderfully strong. The transparent sides of the rostellum, on each side of the disc, consist of membrane, attached behind to the edges of the boat, and folded over in front, so as to form the anterior face of the rostellum. This folded membrane, therefore, covers, almost like a deck, the cargo of viscid matter within the boat.
The anterior face of the rostellum is slightly furrowed in a longitudinal line over the middle of the boat…"
Charles Darwin (1862) On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing. London: John Murray
Darwin describes part of the rostellum of Spiranthes autumnalis as being like a boat (boat-formed, resembling a boat or canoe), but then shifts to referring to it as 'a' this' or 'the' boat (a metaphor). The metaphor gets extended by being said to have a stern and the further simile of a membrane acting like a deck (this could even been seen as an analogy: the membrane on the rostellum is like – maps to – a deck on a boat) which protects the cargo.
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