Where do you draw the Line?

Where do I draw the line?, preferably I don't, at least not on the final surface for my drawing/painting.
I'm defining a line here, as a mark that divides and separates two areas, and in a painting how does that sit with the unified whole.
I had a 'lightbulb' moment while I was drawing Poppy and thinking about her bone structure, I finally realised I should be thinking about masses of colour, tone etc rather than drawing a line and working away from it. Now, if you're shaking your head at my late understanding of a rather basic concept, I perhaps need to add, I did know that there are no lines in nature, I just wasn't applying it.
I am not suggesting I could draw a pet portrait without guide lines because I couldn't, but once my rough drawing is transferred to my final surface then I am going to try to erase the lines to faint suggestions and work in masses. As an example from my own work, I have always defined an animal eye with a hard, black,blue, or brown line. In future I will try to use tiny strokes following the curve of the eyeball to create mass rather than a defined line.
This will become even more important as I continue to move away from pure portraits towards my long term aim of painting animals in scenes. I hope this small step will help my paintings to become much more unified.

Poppy is moving on nicely, I am still working mainly undercoat with pastel pencils and hard pastel sticks. The best bit will be when I add the really soft pastels to hopefully make the painting sparkle.

pet portrait
pastel